ARS Technica reports upgrades to Mac OS X 10.7 "Lion" are far less than perfect for users of last year's MacBook Pro laptops with NVIDIA GPUs due to a driver issue that leads to system freezes and a blank black screen. More details and a solution can be found at ARS Technica.
Numerous reports of crashes have filled an Apple Support discussion thread that sprung up July 20, the same day Lion became available via the Mac App Store. Since then, several crash reports posted to the discussion show a clear pattern of crashes happening in drivers for the NVIDIA GT330M that shipped in 15" and 17" Arrandale-equipped MacBook Pros with switchable Intel and NVIDIA graphics.
Some symptoms of the problem include repeated kernel panics, a screen that suddenly goes black, or the inability to wake from sleep. The specific cause isn't clear, but a variety of graphics-related functions seem to trigger the problems, including switching on the NVIDIA GPU, switching to internal or external displays, and running certain GPU-intensive operations in various applications. In some cases, a black screen can recover by switching between internal or external displays. Some users have also reported success in using the open source utility gfxCardStatus to force a system to use the integrated Intel graphics and/or disabling sleep to prevent the problem from happening.