Plaxo sync has been down for several days. Since we've not been able to resolve the issue with Plaxo. They say we're using their API excessively, but after a fair bit of work on our end (we added special rate-limiting code to our service, used packet sniffing and log file data to analyze our API use) we don't see any evidence of excessive traffic. Until we can correlate what Plaxo are seeing with something we have control over on our end that we can fix, unfortunately Plaxo sync will be down.
Google contact sync is working well, and it's not hard to export contacts from Plaxo and import to GMail.