Lightning (the calendar provider for Mozilla Thunderbird) finally has a release out for Thunderbird 3.0 and all it's sweet, sweet tabs. When I installed it, though, I got the same error that's covering the Lightning add-on project page comments:

"Lightning could not be installed because it is not compatible with your Thunderbird build type (Linux_x86_64-gcc3). Please contact the author of this item about the problem."

I found the bug which essentially says that lightning doesn't provide 64bit builds because they don't have the hardware to develop for it. That's understandable. The problem is there are good contrib 64bit builds out there and they're entirely too hard to find! So if anyone out there Googles that error and gets this post, here it is:

http://releases.mozilla.org/pub/mozilla.org/calendar/lightning/releases/...

That's where the 64bit contrib Lightning builds end up, and it's been working great on my 64bit Ubuntu Karmic 9.10 Thunderbird-3.0 install. Good luck!

** Tip: Go to http://releases.mozilla.org/pub/mozilla.org/calendar/lightning/releases to find a different release.