I’m using Shredder aka. Thunderbird 3.0 beta now for a while and think it’s the best mail client I used so far. I sign all my mails with gpg and sometimes use gpg encryption. However, after my update to karmic (the beta, yes I know, quite a lot of betas), however, Enigmail failed to sign my mails. Interestingly, verifying signatures worked, but signing/encryption did not. The error message was long, telling me that gpg-agent was not able to query the passphrase:

gpg command line and output:
/usr/bin/gpg --charset utf8  --batch --no-tty --status-fd 2
--keyserver-options auto-key-retrieve --keyserver pgp.mit.edu -d
--use-agent
gpg: problem with the agent - disabling agent use
gpg: can't query passphrase in batch mode
gpg: Invalid passphrase; please try again ...
gpg: can't query passphrase in batch mode
gpg: Invalid passphrase; please try again ...
gpg: can't query passphrase in batch mode

After some googling I found a rather radical solution, that, however, solved the problem. I deinstalled gpg-agent, seahorse, and gpgv together which a large amount of packages that are probably quite important for a working system. Then I reinstalled just ubuntu-desktop. And voila, it worked. So, should you experience any problems in karmic with Thunderbird 3.0/Shredder and signing/encrypting mails, maybe try the same trick.