Changing the default screenshot filename

Is there a way to change the default screenshot filename prefix (the Cmd-Shift-4 type screenshot) to something other than the default? The default name looks like this: Screen Shot 2011-08-30 at 10.01.36 AM.png But I'd prefer to use something like: screenshot_2011-08-30_100136.png Basically so I can remove the spaces and junk, since it's annoying to browse dig through files in Terminal. Any easy way to change this prefix with one of the defaults write com.apple.screencapture style commands?