A number of early adopters of the OS X 10.7 Lion upgrade are running into a fairly severe bug. What happens is that Macs will crash with a black screen, which requires a forced shutdown. Some systems may show kernel panic messages on-screen, but the majority of them just have a black screen. The issue seems to happen specifically when certain graphics events are happening such as manipulating images in applications, or triggering interface elements when graphics events are occurring. In addition, it happens when systems are woken from sleep.
Most systems that appear to be affected are 2010 MacBook Pros running a Core i5 or Core i7 processor and with Nvidia 330M graphics, but some iMac systems have also been affected. The problem appears to be an issue with the handling of the Nvidia graphics card, either in the drivers or in the firmware of the card, and may be rooted in how the systems are handling the switch between the onboard and discrete graphics chips in these dua-GPU systems. Affected users have found that if they force their systems to run on the integrated graphics chip then the prevalence of the crashes lessens, if they happen at all.
After a lengthy thread in the Apple discussions surfaced regarding this issue, Apple has claimed to be looking into it, and has asked people for crash reports and other details of the problem. We hope an active investigation will yield a quick fix, but for now the following suggestions may help:
Run on either integrated or discrete graphics
Unfortunately Apple's system preferences have the option to either enable GPU switching or run on the discrete GPU at all times. While doing the latter may be beneficial for some people , if you install the GPU management tool gfxCardStatus, you can force the system to run on the integrated graphics at all times, which should force the system to avoid using the Nvidia drivers. While a few people have still reported crashes when doing this, others have mentioned this has seemed to fix the problem for them.
Do keep in mind that gfxCardStatus may not always prevent the system from triggering a GPU switch, especially during start-up or sleep events, so even with using it to manage your GPU usage you may still experience the black-screen bug, but it should hopefully lessen the prevalence of this problem.
Disable Sleep modes
Since the problem happens for some people when waking from sleep, for now try turning off sleep modes for your system. Go to the Energy Saver system preferences and disable both the display sleep and system sleep to keep the system running in a more consistent state and hopefully avoid any GPU-switching behavior.
If the problem happens to you, pressing the power button to restart the system may seem like the initial and only approach. While this is the case for a true crash where a kernel panic occurs, if the system is just in a prolonged hung state then some other options may also work. These include to wait it out for a few minutes to see if the system starts responding again, but in addition you can try triggering some display activity by pressing the Shift-Control-Eject key combination on your keyboard. If you have an external monitor available, then try attaching that to your system (or detaching it if you are already using it), which should spur a configuration change.
A final option is to enable screen sharing in the Sharing system preferences, and then use a second computer (if available) to attempt to share the affected Mac's screen. If you are able to share the system's screen, then you can safely shut down or restart the Mac.