Most phones use the CPU for the UI. But yes, the latest phones do have GPU acceleration to assist there. But the question is, so what? It's not like the Prime will make a quantum leap improvement in GPU or CPU over the current top dogs like the SGII. Unless it uses some radical new way to implement the GPU in the UI that the SGII and other phones don't use, the UI performance should scale with the current phones. And with 3 times the pixels, that means performance (in this case, responsiveness) goes down.
Sorry, I'm still excited for the phone, but these things have me concerned. Still, I'm glad we're making the switch to 720p. It had to be done sometime for the industry standard to evolve. I'm just thinking that it may take the CPU/GPU world some time to "catch up".