Apple recently issued an update for Thunderbolt-equipped Mac systems that addresses the intermittent black screens some people have been seeing when using Apple's external Thunderbolt display.
The update includes new drivers for all supported graphics controllers from Intel, Nvidia, and AMD, but it also includes components of Apple's OpenCL framework that are specific to the AMD graphics chipset, which suggests this issue stems from a poor software implementation for the Radeon chips in affected systems.
Recently Apple released updates for both Lion and Snow Leopard that address a general black-screen bug in mid-2010 MacBook Pro systems. While the current update is not for these systems, the problem may be related since the issues appear to be involve the same OpenCL framework.
Be sure your computer is fully backed up before applying the update. This is not a firmware update, so if something goes wrong in the update process or if you experience new problems after updating, then you should be able to restore the latest backup.