A few years ago 720p would have been a seemingly unattainable resolution for a cell phone. I think we'll see at least 1080 within the next couple of years. The ipad3 is expected to have a "retina display" and if they follow the trend of the iphone it will be double the pixles in each direction equating to a 2048x1536 display, and surly an android tablet will come out soon after boasting an equal or better display. With ICS tablets and phones will be sharing more technology so I would not think it was far fetched for things go well beyond the 1080 mark for phones much sooner than you might think. Can't the bionic already output a 1080p signal through the HDMI? If so that's still using the gpu, even if it's not the device's screen that its displayed on.
I think 1080p on a screen that is under 5 inches would be as good as any screen needs to be. 1080p output or higher would be nice but I guess that's the only thing that would affect the gpu requirements. Any external or 3D would be where a high end GPU would be required but until the display or harware controllers are capible of better resolutions or 3D is the standard then it would be a necessity. I do like to have the newest however and I am a bit let down about the old GPU. But without hardware that will require more im sure it will last my two year expectancy until I upgrade to my holographic, wireless charging, supercomputerphone.