You can download Thunderbird for x86 Macs here:
Everything seems to work just fine, but I didn’t test it too much so back up your mail often if you’re going to use this. Let me know if you find anything majorly wrong with it compared to the PPC version. Since Firefox for x86 Macs has been downloaded well over 2000 times, we should have a good sized testing community out there… That was a joke, and I’m not talking about the fact that over 2000 people downloaded it. That part is true.
Thanks to Scott MacGregor and David Bienvenu for all of their work on Thunderbird. The upcoming 1.5 release is going to be awesome!