Why not just use Automator's 'Take Screenshot' in a service which lets you specify the name, place etc... (or 'screencapture' in shell script to get a little more control if you prefer) when you actually take the screenshot. I do the former and add '_' + 'YearMonthDay' to file name to make sure they are always sequential. It avoids the desktop mess.

Note: To change screen capture behavior:
defaults write com.apple.screencapture name "New FileName"
killall SystemUIServer

defaults write com.apple.screencapture location "New Path"
killall SystemUIServer

defaults write com.apple.screencapture type [new format] // supports pdf, jpeg, tiff - default is png
killall SystemUIServer

jOhn

Genius, I love defaults write.

The above commands make a of the need for the script redundant (if not all of it).