Unit testing is slowly going forward in Dovecot. In v2.0 tree all the unit tests are now run when you do "make check". They're also run on "make distcheck", which is what I use when building releases or nightly snapshots. So as soon as I start building nightly v2.0 snapshots I should start getting mails every night if the unit tests fail. And I can no longer make a release if a unit test fails. (Unit tests never get run manually - I once fixed a unit test that had been failing for months.)
Besides that, I started trying to add unit tests to lib-index. After some hours of refactoring the code I finally managed to get mail-transaction-log-view and mail-index-transaction to a condition where unit tests could be added to them without too much pain. I didn't really add that many unit tests there yet though.
I'll probably slowly continue adding more tests and refactoring the code more to allow more unit tests, but if anyone else finds they have nothing better to do, now's a good time to add more tests :) Especially lib/, lib-mail/ and lib-imap/ should already be easily testable and they're missing a lot of tests.