"Contacts could not be synced due to inconsistent data."

I just tried to sync the contacts in MobileMe with the Address Book on my Powerbook. The sync didn't work and I got the following message...

Contacts could not be synced due to inconsistent data.
Confirm that your computer's data is valid, then reset Contacts on MobileMe in System Preferences.

I'm sorry but I'm a complete loss for where I go from here. How do I check that the data is valid? What data, the contacts data? Valid? It works on my iPhone and MobileMe accessed on both my Powerbook and iMac.