With the release of Thunderbird v1.5, I also found that I needed to update my version of Thunderbird-Tray from v1.1 to v1.2.
TB-Tray is a launcher for Thunderbird, which stays in memory and will "hide" Thunderbird to the systray instead of the taskbar. It actually has a lot of configurable options for a launcher. I've been using it in one form or another for almost a year. I really hate having my E-mail client show up in the taskbar. It's one of those applications that I almost always leave running and just don't see a need for it eating up a spot on my taskbar.
If you use Thunderbird, I'd highly recommend this application.