Overriding the display dimensions to get a 'round' DPI value helps avoid some problems, see http://bugs.xfree86.org/show_bug.cgi?id=312. Here's what I'm using now (or rather will be as soon as I log out of this sesssion :):
Section "Monitor"
Identifier "TiBook Panel"
DisplaySize 323 216
Option "DPMS"
EndSection
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Earthling Michel Dänzer \ Debian (powerpc), XFree86 and DRI developer
Software libre enthusiast \ http://svcs.affero.net/rm.php?r=daenzer
