I doubt they would come out before the end of the year considering we haven't heard anything yet. I'd love to have a new GPU for my crippling Diablo 3 addiction... I can already feel it coming on!
Since we'll be very lucky to see Diablo 3 in 2011, you should be just fine if you want/need a new GPU for it.
Historically Blizzard has been like Valve on the performance scale, easier to run so that a wide audience can play. Starcraft 2 has broken that trend somewhat, though it is designed to run at a lower FPS ... still, I suspect Diablo 3 will not be too difficult to run, a bit easier than SC2 I would guess. My point being, you probably won't need a new GPU to run it well.