I'm not sure why, but it seems pretty tricky to find a v0.7 copy of the lightning extension for thunderbird on amd64. (Ubuntu Gutsy only comes with v0.5)
This is the version that supports remote calendars and is as such much more useful than the versions that don't.
I had a go at changing the targetPlatform tag in the i686 package yesterday, but that resulted in my Thunderbird being broken and not even displaying mail correctly. Not good.
Anyway, to make a long story short, today I compiled one and please find attached lightning 0.7 for x86_64.
To install in Thunderbird, right-click and save, then open from within the Add-ons manager.