My AppleTV is on a TV with only 1 HDMI port, it shares that port via an auto-splitter with a PS3. This has been the setup for over a year and worked fine. ATV is the default input to the TV, and if i want to watch a dvd on the ps3 i just turn it on. the splitter takes the signal from the ps3, then returns it to ATV when the ps3 is then turned off.
Since around the time of the 3.01 update ATV has developed a bizarre issue, in that every time I turn the PS3 on, AppleTV completely loses its signal. Thats to say, the splitter "returns" the signal to ATV, I can even hear the menu scrolling and clicking but I get no picture, just a black screen.