Apparently I had been lucky in not getting this error prior to tonight. Other HTC Incredible users tend to get it more often.
Google seems aware of the issue but no direct fix appears to be forthcoming, so here is a work-around that should get you through for some time:
On the phone, navigate to Settings > Applications > Running Applications and look for "com.htc.socialnetwork.provider". Clear the data. Your Gmail should start syncing immediately. If not, perform a manual sync.
Your messages should start flowing again.