There is a really cool bulk rename utility
http://www.snapfiles.com/reviews/Bulk_Rename_Utility/bulkrename.htmlhere.[/link] It will let you do all sorts of different file name manipulations on whole directories at once and I just love the GUI.
I started looking for something like this so that I could edit AVCHD clips with Gearshift. I wanted to be able to change all the .MTS extensions to .m2t so that Gearshift would recognize and work with them. Hopefully Gearshift will be updated soon to work with .MTS clips directly, but in the meantime, this utility can get you going.
It is such a great utility, I'm sure it will find many other uses as well.