I don't see GPG/PGP integration. I have been looking for a lightweight, multi-platform offline capable IMAP client with good GPG/PGP encryption for awhile now, and nothing seems to quite fit the bill.
Support for GPG is one of the most popular suggestions for this year's GSoC. I cannot promise anything, but it shuold be there "soon".
I'm also looking for something similar for OSX/Linux. I'd love to move my "life" away from services like gmail (not that I have anything against it, just don't like having all my data that far away from me).
Have you tried Claws mail? :http://www.claws-mail.org/ It has support for gpg (inline and mime) and is multiplatform, maybe you have done and you don't like it
Last time I tried Claws mail (which was years ago) the interface left something to be desired. It was too cluttered, and difficult to use on a small (1024x768) screen.
Unfortunately it's apparently very difficult to keep up with Apple Mail. There's not a public api for the project to hook into, so GPG support always lags new releases of Mail.app by quite a while. 10.8 still does not have a public release of GPGMail.
Drat, sorry. :(
If you use emacs, notmuch and offlineimap are great. Very strong gpg integration.