After doing a clean install of Ubuntu Intrepid last night (which went quite smooth as expected), almost everything kept working, except the combination of Mozilla Thunderbird and Lightning. I've been using this combination for a while now, since I was looking for a good replacement for my calendar software. I think it's a killer combination with the Google Calendar functionality.
At first I was unable to figure out, what might be the cause of my problems, but digging a bit deeper I found out that Thunderbird can help you out with this, by means of the error console.
Try adding '-console' to the startup parameters to enable the error console in Thunderbird. Once Thunderbird is started you can find the console under 'Tools' in the menu.
The actual errors pointed me to this launchpad page. The bug described the same behavior I was having. I was unable to create a Calendar and the Lightning interface looked a bit broken at some places.
Scrolling down to the bottom of the bug report, others seemed to have found a solution, which was actually quite simple. You need to install libstdc++5 separately. You can do this with aptitude or any other package manager.
$ sudo aptitude install libstdc++5
Afterwards you will need to reinstall the Lightning add-on and everything should be back to normal!