Not that I get a lot of PGP encrypted email, but I had GPGMail installed, and I upgraded to Tiger (I missed the chance to do an Archive & Install, apparently - they hid that a bit too well).
Mail complained on startup that GPGMail wouldn’t run, but apparently it still was trying to render the emails through it, as every email I tried to view caused Mail.app to crash. A cursory inspection of the Crash Report told me this was the cause. Disabling it fixed it alright.
GPG Mail has different plug-ins for Panther and for Tiger.
You can download the new one and all will be well.
16 hours, 32 minutes after the fact.
Yeah, I have now.
17 hours, 39 minutes after the fact.