Todd Kopriva posted a clarifying note on the Adobe forums, Mercury, CUDA, and what it all means. There's some confusion between nVidia's CUDA and Adobe's Mercury Playback Engine, which is found only in Premiere -- not After Effects. CUDA support lets Premiere execute some effects (see comments), deinterlacing, blending modes, and scaling faster & better.

And while official Premiere Pro support of CUDA is limited to a few cards, there is a simple unsupported preference hack that has been useful to many with higher end consumer cards with 1 GB RAM or more. One example is a recent report that the Nvidia Quadro FX 3700M is a viable option (realtime playback for several HD layers with several filters), if you don't have a laptop with the Quadro 5000M.

For more background on the the Mercury Engine and CUDA in Premiere, see the 4 reviews in CS5 & CUDA reports and more in other posts on CUDA at AE Portal.