You can also check out: http://osx.iusethis.com/app/metadatamover which provides a powerful Automator action to do renamings, you can use it in your photographic workflow very easilly.

I use this, but I wish it would it would get updated. It is powerful and free, but the counter does not reset automatically: I have to reset the sequential counter manually in the "Edit Styles" menu INSIDE the preferences. This is very clunky, but I put up with it because it does everything else beautifully.

Very easy to use and it works as promised. A+
I've renamed over 1 gigabyte of photos in about 45 seconds!

This is a very handy application. I use it to rename photos of my daughter, which I keep in a single directory. I use this program in conjuction with Image Capture.