OK, so I installed the Lightning 0.9 add-on to Thunderbird in order to have Calender functionality, and I didn't notice until yesterday that I can't actually add any events to the calendar! The buttons and menu options are disabled.
This evening some quick googling turned up this reference to Bug# 278853 which is actually misleading because this isn't a bug in Thunderbird at all, but a dependency of Lightning 0.8 and 0.9 on a library that isn't installed on Ubuntu 9.4 by default.
- Uninstall Lightning 0.9 from Thunderbird's add-on dialog;
- Go to Synaptic Package manager and search for "libstdc" and select to add the libstdc++5 package and apply changes;
- Re-install Lightning 0.9
Result: I can now add an event to my calender reminding me to record the show "Being Human" on BBC America on the evening of July 25th.