When attempting to save some changes to one of my Mail.app IMAP accounts in Leopard, it refused to save as I "already had an account" with that name.
Great, of course I did -- it was the one I was editing. The error was no more helpful than that. Taking a look inside ~/Library/Mail to see if there was old garbage left behind causing conflicts, I noticed CalDAV accounts set up in the new version of iCal were also stored here. And, of course, one of them had the same name.
Renaming the CalDAV account from within iCal's Accounts preferences then enabled me to save my settings within Mail.app without error.
So it seems that, oddly, Mail.app and iCal share an account namespace for entirely different functionality. I haven't found any mentions of this anywhere else, so hope it's useful for somebody else's troubles...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071113202010496