Make sure you're running libsdl >= 1.2.14
http://www.libsdl.org/release/changes-1.2.html

SDL_GetVideoInfo() can only be as wrong as your X modelines, if you're using 
xinerama this is a well documented problem (with SDL).

this may be helpful:
http://forums.libsdl.org/viewtopic.php?t=7233

also, try setting SDL_VIDEO_FULLSCREEN_DISPLAY=0 and running