I'm making a cake with whipped buttercream filling and poured ganache icing- do I need to refrigerate it? I plan on decorating it with fondant decorations that I've already made and dried so they can go on at the last minute.

why would you refrigerate? ganache shouldn't go in the fridge. Nor should fondant.

I was thinking that it would help it firm up. Also- now I've got another issue- the ganache seems to be sliding off the sides and not adhering at all to my cake! Any thoughts?

What kids of buttercream is it? If it is a SWBC or a IMBC or a French buttercream? I undertand that if butter cream contains any egg it in it requires refrideration to be food safe. It can only be left out of fridgerator for a couple of hours . I saw a chart that was Wiliton that was supposed to talk about the different icing requiremnts it may be of some help. http://www.wilton.com/cake/icing/icingchart.cfm. Finally If you have any questions I would ask your local health inspector for what is required in your area. Good luck!

when i pour ganache for my doberge cakes I always fridge them to firm up.