I currently run OS X 10.5.6 and my TeX distribution is TeTeX from MacPorts.  I recently found out that my pdflatex is generating A4 paper by default, even if I use the letter option with \documentclass.  Solution: call texconfig-sys and choose Paper -> Letter.  Unfortunately I couldn’t get it to write locally to my home directory — I set TEXMFCONFIG to $HOME/Library/texmf but texconfig-sys didn’t seem to care.  That’s unfortunate, but I am the only user of the computer so I just set it locally.