If SC-2 came out tomorrow, i'd vote 2D. But since it isn't, i voted 3D because it'd just be a nice change to the 2D graphics we see today on SC & BW, and hopefully my computer will be good enough by then to run SC 2.
Also, i doubt Blizzard would make SC-2 2D, because honestly, the gaming media would critisize Blizzard for making a game that is so similar to it's predecessor, and Blizzard wont want that. So i reckon we should expect a 3D game similar to WC3, if we are to expect a SC-2 at all. Besides, i like the fact that WC3 only has 90 food supply, so you have to rely on awesome strategies and micro-management. Hopefully they don't install invincible heroes that can wipe out an entire army :rolleyes