A command-line utility included on the Apple TV OS at /usr/sbin/screencapture that can be used to take screenshots in Mac OS X. This would be useful for documenting the hacks and other activities on the Apple TV. Untried and untested in the Apple TV OS, but should work with:
PNG is the default. Other file formats (TIFF, PDF, JPEG, etc.) can be selected with the -t switch. See man screencapture for more information.