In the past days I experienced errors in my anti-spam plugin by Jay Allen on my old MT installation. Turns out that the ban-list of spammers to filter on became too long. The original ban-list plugin stores all ban-listed entries in one database record, which over time becomes too long. Newer ban-list versions have that solved but require a newer version of Moveable Type.
Bottom line: Adding three new domains to my ban-list turned into buying and installing Moveable Type 3.2.
Upgrading went without a glitch.