Some simple modification to a Premiere CS5 preference file and to Nvdia preferences can enable CUDA acceleration on more Nvdia cards, according to reports on Premiere forums. This is not driver modification (softmod) or overclocking, but it is unsupported.
The mod seems to have popped up at DVinfo.com in How to make Premiere CS5 work with GTX 295 and possibly all 200 GPUs by marvguitar/Martin Guitar, and spread elsewhere, including to the Adobe forum at Unsupported GPU for CS5.
On Windows, the "GPUSniffer.exe" looks at "cuda_supported_cards.txt" to match the named list of cards with your GPU, so it's just a matter of enabling your card and seeing what works. Adobe has made details on CUDA acceleration and supported cards pretty clear from the start in order to create a pipeline with higher performance, quality, and reliability guarantees. There are plans for more cards to be certified in future but it will take Adobe and Nvdia time and testing resources to deliver new certifications.
If people want to risk some incorrect renders and crashes -- including possible system instabilities on cards with less memory -- then they'll have to take responsibility themselves for testing and support. For jobs, using an untested uncertified card is not advisable, but you should be able to turn CUDA off and render in software. Having inexpensive hardware acceleration for a few layers with effects on DSLR footage has made some users pretty happy.
Some cards will work better than others, but to find out you may have to enter the confusing world of nVidia naming conventions and driver support for CUDA. Here's one list of cards (by no means exhaustive or accurate) that are reported to work:
GeForce GTX 260
GeForce GTX 275
GeForce GTX 285 ( "3 layer limit", Adobe, Mac)
GeForce GTX 280
GeForce GTX 295
GeForce GTX 470
GeForce GTX 480
Quadro CX (Adobe)
Quadro FX 3800 (Adobe)
Quadro FX 3700
Quadro FX 4800 (Adobe, Mac)
Quadro FX 5800 (Adobe)
Update: appearing in late June twitters were of an article by Studio 1 Productions, How to Unlock Adobe Premiere CS5 use almost any NVIDIA graphics card with CUDA acceleration.
Update 2: Premiere gets laptop support with the addition for the Quadro FX 3700M and the Quadro FX 3800M to the list.
Update 3: there are some instructions for the same on the Mac at Insanely Mac in How To Cuda / Mercury Engine on Premiere CS5 / Snow Leopard [ lower end graphic cards ].