To invoke, use the follow default hotkey: You can change this hotkey to anything you want by editing the workflow.

Control+Alt+`

• In the Alfred dialog, press a number from 1-5.
• You can also clear the current rating by pressing 0.
• Inputting a number less than 3 will automatically jump to the next track

You'll notice I've put in a "Clear iTunes Rating." This is performed to pass the iTunes current track information to the Growl notification. This runs before your rating is applied. If you have a better hack or know how to pass it using AppleScript, do share!