Did this start at about the same time that the code for wrapping div tags around sigs was added? Do you have anything in your style sheet that might bork it?

Also, I recently got to play with OSX (10.2.6 iirc) for a few days. I installed Opera on it and it seemed to work pretty flawlessly. You'll have to use Opera 6 because they haven't released Opera 7 for that platform yet, but it'll be worth the wait. Opera 6 is still very functional. Just lacks some of the customizable features in 7.