So after unsuccessfully lobbying our IT department to turn off Public Folders when the Exchange server is accessed through IMAP, I took the plunge and installed Thunderbird 1.5 for OS X. It actually works pretty well (which means: it doesn’t peg the CPU when updating imap folders, LDAP lookups work, even with substring searches, and it doesn’t have the weird UTF-16 encoding bug that Ramesh complains about on Linux).
Since I have to start up the VPN client to connect to my work anyways, it isn’t too much trouble to have to start up another app to get to my work email.
The threading behavior is still better in Mail.app, specifically, I want it order threads by the newest message in the thread. So old threads with new messages show up as recent.
Also, there doesn’t seem to be a way to default to plain text when composing a message. I looked in the advanced config editor, and found an option that seem to indicate that it would turn off HTML composition, but it doesn’t seem to work. Oh well.