With active proof-of-concept exploit for Linux. This is not only a local vunerability with X + nvidia drivers, but it can be remotely exploitable.. for instance if your browsing to a malicious website it's possible for somebody to have your browser display something _which_could_give_them_root_access_.
This is a problem with how Nvidia accelerates rendering of text. This is a very very serious problem.
This bug has been around for years now. First reported in 2004 it took nearly 2 years for Nvidia to aknowledge the problem, which was in July 2006 and they still haven't fixed it.