I.heart.macosxhints.com. An enterprising reader of their site submitted an AppleScript that offers you a simple dialog for each .Mac alias you have, allowing you to enter custom names to be associated with each alias in Mail.app (something Apple badly, badly needs to add themselves). It works wonderfully, but here's a tip: make sure Mail.app isn't running when you use this script, as it's doing nothing more than editing Mail.app's preference file in order to add these names to each alias.
Here is a link to the AppleScript, which I recommend you simply copy into a new window in Script Editor. Enjoy!