I'll say it again.
If someone else comes out with a working, completely open-source 3D driver, I'll switch over to their product in a heartbeat. If nVidia does it first, I'm staying with them for as long as I'm in this market.
I tried switching to the Radeon 7500 and DRI early last summer, but the card I got had issues (it was running Q2 slower than my GF2 GTS), so I moved back.
But an open driver is possible. Put anything that absolutely has to be closed, on the card. Then publish the interface to it.
Oh, and I'd think about definitely getting rid of the ability to overclock the card through the drivers. While this is nice for some, you WILL Have people demanding money for the cards they've broken by doing it if you go to an open driver. Their claims may not hold up in court, but it'll be much easier if you just get rid of that feature.