I assume you mean a real fountain with actual water... What about having a hole in the middle of the (top) tier with the fountain up through the hole so the water pool level is level with the top of the cake and it looks like the fountain is part of the cake?
The fountain is making a return appearance? 
Don't wish to offend anybody but those cakes have to be some of the ugliest I have seen
Can't imagine how you could make it look good - but if that's what she likes - then she will probably be happy with the original version! LOL
So pleased they never took off here in Australia 
The fountain is making a return appearance?
Don't wish to offend anybody but those cakes have to be some of the ugliest I have seen
ditto
Being one of those oldies who cut her cake decorating teeth on fountain cakes ...... 
I think the clear stands look much MUCH better and more modern than the older style of the white plastic. Decorating the table around the base with LOTS of flowers/petals also helps soften the look.
I would also say "be prepared". In my observation, you can tell the state of the economy by the size of the cakes. The worse the economy, the bigger and grander the cake (just like how lipstick sales go up when the economy goes down!)
During the late 70s, early 80s, at the peak of the Oil Crisis, fountain cakes with stairs were THE big thing. As the prosperous 80s rolled in, fountains moved out and the (what I call) the short, dinky stacked cakes rolled in. A few years ago, when gas first hit $4/gallon, I saw an increase in the requests for cakes with pillars and/or fountains (did a few myself). As gas dropped back down to the $2.50/gal range, the short dinky stacked ones moved back into popularity.
gas is back up to $4/gal ..... cakes are going to get taller and grander.
My unscientific reasoining is that people are very willing to save and cut back and do the rice-n-beans things for everyday living. In the 70's/80s, we were willing to turn our furnace down and drive tiny fuel efficient cars. But during tough times, people are "tired of it" and seem to think "Its my wedding day ..... I'm not going to have a 'fuel efficient' wedding!" They want something, not necessarily mroe expensive, but something that LOOKS grander and bigger.
Sorry for the detour with the history lesson, but I found this observation to be interesting!
I had a bride ask about one the other day and this was the photo she brought in:
http://weddingnuance.com/2011/02/wedding-cakes-gallery-with-fountains/fountains-wedding-cake-2/
Hey! I did a scaled down version of that cake last August! http://cakecentral.com/gallery/1437261 I wish the bride had left way more florals than she did (would have like more around the base of the big cake and more "fill" around the fountain.) But you can see how I used the clear pillars on the smaller cake and I like that look much better.
coincidentally, I went to facebook and CCer jillmakescakes posted this photo of a fountain cake she did recently.....
http://www.facebook.com/#!/photo.php?fbid=10150181971006684&set=a.344806026683.155465.334033731683&type=1&theater
I too just had a very young (20) couple bring me a pic similar to the one posted above but I believe was actually created in 1987!. I tried so hard to talk them out of it with no luck. She is et on the fountain, the stairs, the pillars, the little plastic people, ect. I dont want to do this cake!
Here is the only modern-ish one I can find.
http://www.weddingred.net/wp-content/uploads/2010/07/fountain-wedding-cake1.jpg
http://www.outrageous-cakes.com/sitebuilder/images/Fountain_Cake_6-236x323.jpg
Maybe make the cake look like a real, outdoor type fountain? There does not appear to be water in this one. It looks like they used monofilament. If she wanted lights, maybe you could get some fiber optics, or just add some colored led lights to the holder.
http://farm5.static.flickr.com/4069/4633423214_f0f95a974e.jpg
Maybe the fountain itself has to be less traditional, newer or more interesting looking.
Such as the 'fantasy' type here.
http://www.sugarcraft.com/catalog/wedding/Wfountains.htm
Here is the only modern-ish one I can find.
http://www.weddingred.net/wp-content/uploads/2010/07/fountain-wedding-cake1.jpg
http://www.outrageous-cakes.com/sitebuilder/images/Fountain_Cake_6-236x323.jpg
Maybe make the cake look like a real, outdoor type fountain? There does not appear to be water in this one. It looks like they used monofilament. If she wanted lights, maybe you could get some fiber optics, or just add some colored led lights to the holder.
http://farm5.static.flickr.com/4069/4633423214_f0f95a974e.jpg
Maybe the fountain itself has to be less traditional, newer or more interesting looking.
Such as the 'fantasy' type here.
http://www./catalog/wedding/Wfountains.htm
I like the 3rd one that is really pretty
Wow--you guys get up early!!! Thanks so much for the help. I too am not looking forward to this cake--at least she doesn't want the stairways and people-ugh!! The wedding is at a fancy hotel in Minneapolis, and when they see me walk in with a fountain cake, I'm sure they'll think "grandma stuck in the 80's"
I do have the clear plexiglass stand(not Wilton) buried somewhere--at least that should help. Maybe I'll luck out and they won't have electricity at the cake set up location!!
"stuck in the 80s"! 
Bear in mind that before fountain/stairs cakes, wedding cakes mostly looked all the same .... mostly stacked or some with pillars; all white cake with all white icing; maybe some colored icing roses. All. Looked. The Same.
When fountains came out, they were such a WOW factor ..... something truly different (like those nasty looking zebra striped TT cakes that I really dislike! 
) ..... and everyone wanted them. It was the only cake I made every single weekend for at least a year.
ONE time during this period I had a bride request a stacked cake and everyone had the same reaction as you "young 'uns" do now ...... "Wow, look at that old fashioned cake! Where did THAT come from? Haven't seen one of THOSE cakes in awhile!" 
what's old is new and vice versa!
(ahhhhh! I luv sharing history!)
Indydebi you just crack me up!! kinda like the same cake I've been doing for years-a few stacked layers with the colored band around the bottom of each tier and then a few flowers or maybe a dot. "Something simple and elegant" they always say yet they spend thousands on decorations. I'll definitely tell everyone that it's the "latest"-hehehehe.
Jeannem, those would be the brides who say "I want something DIFFERENT! can you do a dinky stacked cake with ribbon around the bottom and these scrolls on the side?" (tilt head, bat eyelashes, look dizzy.)
And then we have to give a big sigh, assure them that yes, we can manage to make that cake ..... again. then we close our eyes and do it in our sleep.
Yeah. "Different". Luv it.
Well based on the WOI factor - we might JUST be safe here
Our economy is thriving..... so US Aussie cakers are left with "under-stated fondant elegance' INSTEAD of those hideous plastic trashy fountains.... often containing the ALL IMPORTANT.... coloured water !
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