Since my post, I have installed gnustep and windowmaker and am loving it. It's snappy, fast, and lightweight. I'm also partial to the vertical taskbar setup which WMaker seems to favor.
GNUMail.app is actually a pretty slick email client, and it of course is perfectly integrated with GNUstep. If you're using GDM as your login manager, you should be able to select WMaker from the "Session" menu. Good luck!
