I spent a few hours I won't get back, too. Looks to me like it's not gonna work with current versions of software.

The lamest of workarounds for now, at least it's something, is getting the unoficcial MoreFunctionsForAddressBook extension for Thunderbird. On the N900, export all your contacts as vcard 3.0's. Now in the Thunderbird Contacts, use Tools > MoreFunctionsForAddressBook to import them all at once.

People are working on getting Thunderbird to synchronize with Ubuntu One for Ubuntu 11.10. Until then, you can use the unofficial Hedera Contact Sync extension. But it's so alpha and new, I haven't got a clue why it works on one and not on the other.