XBOX 360 and PS3 both supply a HDMI signal.
Apple’s cinema displays only accept a digital DVI signal.
There is no way to convert the HDMI signal from xbox into a DVI signal.
One company (forget which one) sold a special HDMI-to-DVI converter box for a bit, but they’re no longer selling it and aren’t intending to start selling something like it again.
If you’re buying a LCD computer display, buy one that has a HDMI input, or at least composite or VGA.
(figured I’d put this out here since it’s the 2nd time someone asked me, and I remember being really frustrated when I couldn’t get it to work)