As previously detailed, I've had problems synching my Sony Ericsson K770i with Ubuntu. In fact, I gave up and did it via a temporary Windows install.
It's niggling now, that when I want to add a quick address I simply can't synch the Thunderbird email client with the phone.
My first plan was to simply export the new contact from Thunderbird in the vCard format and Bluetooth it over. But Thunderbird doesn't export as vCard :(
Enter the not-so-catchily-named MoreFunctionsForAddressBook extension, which adds a slew of features to the Thunderbird Address Book, including export as a vCard. Just right-click and Save link as to your desktop, go into Thunderbird -> Tools -> Add-ons -> Install. Once restarted, select your contact in the Address Book and Tools -> Actions for contacts -> Export as vCard. Locate the vCard and Bluetooth it to the phone.