When I launched Adium this morning it was kind enough to tell me that version 1.0.1 was available and asked if I'd like to install it. Of course I would, thank you. Once the install was complete, I was also asked if I'd like to allow the new version to use all of the previous Keychain information. Yup, so I clicked “Change All” and waited… and waited. My MacBook started thrashing so badly as to be unusable.
Long story short, after farting around for a couple of hours, this post at unsanity.org had the answer.
sudo mv /var/db/CodeEquivalenceDatabase /var/db/CodeEquivalenceDatabase.old
open /var/db (and then manually move CodeEquivalenceDatabase to the trash.)
Seems securityd will flail about gobbling up memory and getting rid of CodeEquivalenceDatabase clears things up nicely.