There is a new update for Premiere Pro CS5 that's available for download: version 5.02. It claims to fix a lot of bugs that version 5.01 suffered from. Plus, in addition to the GeForce GTX 285, Quadro CX, Quadro FX 3800, Quadro FX 4800 and Quadro 5800, Premiere Pro CS5 v5.02 now also officially supports the GeForce GTX 470, Quadro 4000 and Quadro 5000 (all three of those are based on the newer NVIDIA Fermi GPU architecture) in the Mercury Playback Engine's GPU acceleration feature. And due to the originally supported GPUs being either EOL or astronomically expensive, the GTX 470 (around $330) is now the least expensive Adobe MPE-certified GPU.
Sadly, MPE's GPU acceleration feature still does not support AMD (formerly ATi) GPUs for now. Thus, AMD GPUs are still stuck in MPE's software-only rendering mode.