As long as I can remember - at least since Mac OS X 10.4 and possibly even 10.3 - Mail.app hasn't been able to maintain column widths to save its life. It doesn't matter if I never touch the window width, or if I manually set each column exactly where I want it, or even if I ask nicely - Mail.app almost seems to enjoy playing an obnoxious game of 'what me worry?' with my column widths, tossing them to the digital wind at every opportunity. Run a search? Hello suddenly-double-sized-Attachment-column. Put on a pot of coffee in between TUAW posts? Better make extra room for the Date Received column, cuz there's no telling how much space it's going to gobble up behind my back.
Ok fine, for the record: I don't actually believe Mail.app can tell when I'm making coffee. Still, the point stands: Mail.app needs to brush up on the 'How to be a Good Lil App' rules and learn how to keep everything in its place.