If you get a ton of email and don't want to be distracted by the bright red "47 unread messages" overlay on the Mail.app dock icon, I finally found a way to disable it. There is probably an even better way, but this seems to work well enough for me. Control-click on Mail in the Finder, and choose Show Package Contents. Then navigate to Resources, and rename the file called newMailBadge1&2.tiff. You could change the name to anything you want, but I suggest newMailBadge1&2.tiff.orig.
This doesn't completely disable the way Mail.app writes the message count onto the icon, but the contrast is reduced so much, you'll no longer be distracted by a big red dot taunting you to waste more time reading email.
[robg adds: Make sure you quit Mail first; it's a good thing to do when you're modifying a file within an application bundle.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050125033232800