Running Thunderbird 2.0.0.9 and Growl Notifications 1.0.2 with Growl 1.1.2 on OS/X 10.4.11
If I receive a large amount of mail at the same time (in the mornings I tend to have 50-200 new messages) GrowlHelper stops responding, and chews up an entire cpu until I go and forcequit it.