Ahhh, some nice DRY evening reading.. The Atari 400/800 hardware manual. Continuing my quest for getting a grasp on the whole system... I found this paragraph in section 2 slightly amusing....

Theoretically, it is possible to write a program which changes graphics or colors "on the fly", i.e. during the middle of a TV line. However, with all the DMA going on, the cycle counting gets to be quite complicated, and is beyond the scope of this manual.

Translation: shit man, we know it's possible if you're god's gift to 6502 assembler, otherwise man. Sheesh, we don't know. *Shrug*