Please keep the following things in mind:
- Plaxo stores the address data on remote server. You may want to read their Privacy Policy first.
- The contact's primary e-mail address has to be mapped as "Work" in Address Book.
- Backup your Address Book data first! Due an mistake, I accidently trashed all birthday entries. (A modification log is accessible through plaxo.com though.)
[robg adds: This previous hint explained how to use Plaxo to migrate contacts from Outlook to Thunderbird. I don't use the service, so I haven't tested either hint.]

