The new nightly release of Thunderbird (10.7mb download) sees the inclusion of Kevin Gerich's wonderdul pinstripe theme into Mozilla's Thunderbird. It's a wonderful app, and is now my default over Apple's Mail.app, but it does still have some omissions. For instance, the address book doesn't use Apple's standard Address Book, and you can't import Mail.app's mailboxes –- until now. Before trying the following, make sure you back up all your data!
Go to your Mail.app Library folder, usually located in your user's Library -> Mail -> *your default account*. Make a copy of the inbox.mbox (perhaps to the desktop), then control click on the copied inbox.mbox and select Show Package Contents. Locate the file called "mbox" and change its name to "inbox." Run Thunderbird, and set up an account (the same as the account in your Mail.app).
Close Thuderbird and navigate to the default user's mailbox, which is normally in your user's Library -> Thunderbird -> Profiles -> Default -> [random_string] -> mail -> *your mail account*/. Overwrite the inbox file with the file you've just changed in the Inbox.mbox. This worked for me, although I can't confirm any attachments. I guess you could do this for all your accounts / drafts / outboxes / archives as well, to restore all your old emails. Obviosuly if it fark's your machine - I take no responsability, but I hope it helps some people. I'm now going to convince my director we should be running TB on all clienst (open-source rocks).
[robg adds: I'd love to say I tested this, just to check out Pinstripe, but the latest nightly builds won't run at all on my G5. I tried the last three or four, and they all failed -- even after trashing all Thunderbird-related files on my drive. I'll have to test later on another machine...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040401075611985