This was emailed to me by Peter P., and I think it's been published a few other places now. If you don't want "US Letter" as your default paper size, here's how to change it.
If you want your page layout to default to A4 size, instead of letter, just paste this into Terminal. It will copy the British resources to the (American) English location:
sudo cp[space]VERY IMPORTANT NOTE: I've broken this onto SEVERAL lines to keep the story narrow enough to read! Do NOT copy and paste the entire section. Replace the '[space]' words with an actual space character, and enter this on one line. There are ONLY two spaces in the entire string after "cp", as shown above.
/System/Library/Frameworks/ApplicationServices.framework/
Versions/A/Frameworks/PrintCore.framework/Versions/A/
Resources/en_GB.lproj/Localizable.strings
[space]
/System/Library/Frameworks/ApplicationServices.framework/
Versions/A/Frameworks/PrintCore.framework/Versions/A/
Resources/English.lproj/Localizable.strings
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011018124937144