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.
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.