(yet?)
Never.
As for performance, it depends a lot on the drivers you're using. Under Linux, ATI's drivers are faring not-so-well whilst nVidia's drivers are god-like (seriously, they did everything right). A quick Google could probably produce a lot of problems with either set though, as there's just no easy way to predict problems. As far as I'm concerned, ATI has a primary interest in Windows and will support their Windows drivers better and not give much thought to Linux.
Just my opinion. Anyways, I've got my GeForce 6800 Go working under Linux and it works like a charm. Everything is speedy and runs well -- I haven't compared it to the Windows speed, but I'd assume they're the same, Linux may or may not be faster depending on what modules, daemons/services, programs, etc. are running though.