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 53.23MB update is for systems running Snow Leopard, and should be available via Software Update (in the Apple menu), but can also be downloaded from the update's Web site.
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 bothand that address a general . 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.