Out of all the games I have tried on my PowerBook, only one knew of the 1280x854 resolution of the widescreen display. Specifically Warcraft 3. All others I have had to edit a settings ini or plist file to manually enter the resolution.
So far the following have worked:
Unreal Tournament 2003/2004 - including non distorted widescreen support
Shadowbane - Including flawless widescreen support, no interface distortion either
Warcraft 3 - Displayed widescreen, but clearly just stretched
Halo - Can't remember how it dealt with it, since the game is so sluggish
Command And Conquer Generals - Haven't tried a widescreen resolution. 800x600 and 1024x768 stretch to fill the screen