Just talked to a CM at All Star Movies in regards to a birthday cake.
As of today, March 14, 2012:
You can get a basic white or chocolate Celebration Cake at the food court (available all over WDW). It is not and cannot be personalized. Cost is minimal - under $15 - probably $12.99 like other poster mentioned.
Custom cakes are available. Call WDW Cake hotline: 407-827-2253 and follow prompts.
You will not be able to catch a live person (I've tried), but they've called me back within a day both times.
At this location specifically, as of now, they have cakes in 8" or 10" round, and 3 sizes of sheet cakes available.
Any range of characters (including Winnie the Pooh & friends - not all resorts have that option - I know Contemporary does not)...
8" cake is $32 plus tax and includes everything:
piping in color of choice
cake flavors: vanilla, chocolate, white, german choc, red velvet, carrot;
frosting: buttercream, fudge ganache, cream cheese, whipped cream;
filling: frosting, raspberry, strawberry, fudge, etc.
transfer image (of character(s) of choice) if you'd like one
inscription (color choice)
The cakes are made by a local bakery and delivered to the World Premiere Food Court. You can pick up the cake and take it to your room if you wish. They will provide plates, forks, etc. (only have plastic knives - no servers). They even have candles and matches if you'd like.
Biggest issue I see with this is that they have absolutely no way to show you or tell you what your cake will look like. CM assured me all the cakes she has seen looked very cute. I am a little skeptical after looking through the photos on this board though - some are cute - others are not "Disney quality" in my book...but the dates and locations of those photos are not all mentioned, so this is at your own risk I guess. My thought is that bakeries must have licensing rights for Disney characters, so I plan to check my local bakeries books for the designs available and hope that's what I might get.
Order up to 2 weeks in advance, must have at least 48 hrs notice - that's standard everywhere.
Keep in mind each resort and restaurant has their own unique options: Chef Mickey's had completely different options and pricing than All Star Movies... I don't have my notes on me for that location, but very different.
I think you've already gone there by now, but hope this post helps others.