the mail application on my ibook recently stopped working. i think it was a combination of moving it from the applications folder to a sub folder, and then upgrading it when i upgraded to OSX 10.2.1. when i tried to run it, the app just bounced a few times in the dock and then vanished. the console showed error messages reguarding a missing nib file. anyhoo, after a bit of poking around i discovered Pacifist, a util based around the unix pax util, something i'd been looking at recently for something else. using this i opened the essential packages package from my OSX install CD, and was able then to re-install the mail app.
and it works. i am now of course in the position of having most of 10.2.1 installed, but the mail app from 10.2.0. it seems to work ok though, so it's not something i'm particularly bothered about.