I am so excited! I finally did it! Just wanted to share with you all how I did it. (thanks to a cake contest show for the inspiration!)
It will glow under a black light.
I used clear piping gel. Heated that in a sauce pan. Added tonic water (the ingrediant that will make it glow) I used this for slime on a Ghostbusters cake so I used enough tonic water to make it very thin. I added my coloring at this point. After heating it very well I added plain gelatin. Let it set for 2 hours and it was thick enough to spoon over the cake for slime.
The possibilities are endless. I made another batch later on that was thicker. Like the piping gel that wilton makes. A side note tho, the slime appeared to "eat" the fondant after a few hours. It is something that you want to wait to put on.
I loved doing this cake. It was a mess but so fun. The boys loved it!
I couldn't attach a photo so here is the link for the Ghostbusters cake I posted today. 
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1454532
Your cake is awesome! Thanks for sharing your discovery. With Halloween come up the ideas are swarming in my head! How fun. Can't wait to try it!!!
Wow! So cool! I saw Mary Maher do that and wondered how it worked. Thanks SO much for posting all the info. I can't wait to try it! 
I tried to do that for a quinceanera cake I did yesterday and it didn't work for me. I only used the piping gel and the tonic water. I guess it needs the gelatin. I will try it like that next time though.
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