Kirk Samuelson has directions for you if you are looking to rename mass files in the Terminal in OS X.
It's a nice command, just saved me a ton of time. I was going about it wrong before, trying to mv *.old *.new. When really what I needed to be doing was % ls -d *.html | sed 's/\(.*\).html$/mv "&" "\1.php"/' | sh
Nice.