Björn comments on my previous post:
The default 60Hz-because-of-TV-refresh makes me wonder: does this change to 50Hz when the XBox is connected to a PAL TV? Or - from a not too serious POV - is this just another default chosen in the spirit of "We are Americans and deal with i18n issues in v.Next"? :]
For American readers: PAL is the television format used by the rest of the world. It runs at 50 hz, as opposed to the 60 hz of the American NTSC format (except for Brazil, which confusingly uses PAL but runs it at 60 hz).
This obviously makes no difference if you are using variable timesteps: your game will work the same whatever refresh rate the output device may be using.
For fixed timestep games, things can go a few different ways.
In the bad old days, some games used to just change their fixed timestep to 50 frames per second when running on a PAL device, making the gameplay run 1/6 slower as a result. European gamers who remember the days of the SNES and Megadrive will be all too familiar with crappy ports of Japanese games that felt sluggish as a result of this!
More polished games used to change their fixed timestep to 50 frames per second, then adjust all their speed constants to compensate. Old-skool European gamers will remember the frustration of waiting six months for a PAL version after games were released in Japan and the US. Part of that delay was tweaking the gameplay to work at the new framerate.
Modern games often leave their fixed timestep set to 60 hz even on a 50 hz TV. This keeps the gameplay the same with minimal effort, at the cost of a slight visual stuttering. The resulting glitches and tearing drive some people up the wall, but others will never notice it: your mileage may vary.
Fortunately, this problem is in the process of going away entirely. Modern PAL televisions are capable of displaying both 50 and 60 hz signals, so you can run at 60 hz and know every gamer with a decent TV will be able to see your game as it was intended, no matter what continent they may call home. People with older televisions will see some stuttering, but hey. If it bothers them, that's a great excuse to splash out on a new widescreen plasma display :-)