As seen in this thread, Growl has stopped showing notifications completely, and at Peter Hosey's suggestion, I enabled logging and after looking at the log, learned that there wasn't enough screen space available:
However, I'm on a 1440x900 screen with plenty of room, so I don't know what's wrong. It would be great if any of you knew what the problem was!
I haven't turned on logging so I don't know if I have the exact same issue, but for me Growl has just stopped working since Leopard. Period. No notifications, nothing. I have re-installed it but no luck...