Complaints run something like this: When the more powerful Geforce 9600M is switched on, the display is ravaged by green lines and other weirdness, which gets worse as the card appears to heat up. Switching to the integrated 9400M cures the problem, as does restarting (at least temporarily). Another user notes that the problems flare up doing gaming, but calms down when they're "doing lower-performance stuff" which also seems to point to a heat issue.
This is actually the third run of MacBook Pros to be allegedly affected by defective Nvidia graphics cards with heat problems: The previous generation was packed with Nvidia cards that Apple determined were suffering from its notorious material defect that caused cards to fail at a "higher-than-normal" rates. The current generation of 15-inch MacBook Pros also had video problems (the ominous "black screen of death") which Apple also acknowledged, though no definitive blame was ever placed on faulty cards. (Although the Inquirer will readily tell you the cards are in fact, defective.)
Are these graphics cards, in fact, bad? It's too soon to tell—and very possible we're just hearing from a tiny, tiny but vocal minority that's been afflicted—especially without definitive word from Apple. I'd say, as I did before, that given the not-so-distant past, it doesn't look very good for Nvidia, whatever the actual case may be. [Apple via Engadget]