Plaxo has just pushed live their MicroID implementation at myplaxo.com.
claimID has the first announcement:
Another notch on the MicroID belt.
Today we add Plaxo to the list of Known MicroID Publishers. Welcome, Plaxo! And thanks Joseph!
You can see it in action in the head of any page with the pattern http://NICKNAME.myplaxo.com/.
Plaxo will create and publish a MicroID for each of your verified email addresses in your account. You need to visit your settings page and claim your NICKNAME.