GREGWELCH My apologies. Version 1.0.1 (just released) should fix the problem.
What happened? The plugin was erroneously relying on something in the Mail Act-On plugin, so anyone who did not have that installed, experienced a crash. I actually did have several people help me test the plugin before I released it, but unfortunately all those people had the Mail Act-On plugin installed, and so none experienced the crash. 1.0.1 was also compiled as a universal binary, but not tested on an Intel machine (I don't have one).
--Greg Welch (the MailFollowup developer).
(Version 1.0.1)