Since Plaxo decided they wanted to be just another social network and leave contact management behind I decided to look for something else.
First you will have to sync up your contacts with Google Contacts. They launched their API in April and all Plaxo came up with was a one-way sync, pff... I was syncing Plaxo to Thunderbird, so I got this add-on called Zindus which did a nice job syncing some contacts that were already on Google Contacts. You'll end up with a ton of work to do. My 500 contacts became 800, but they were not mostly duplicates. Most of them were people I had on orkut so Google added them automatically to Contacts.
I'm planning on getting an Android based phone so this would end here, BUT since I don't own one yet here is how you'll sync: Soocial. Soocial can sync your Google Contacts with a bunch of phones and Mac. Enjoy!