ZCM on W2K3 SP1 using AD as the user source.

I had the agent deployed to 3 workstations and was able to login OK.
Last night I set up a deployment task to push the agent out to 9 other
workstations. This morning I couldn't log into ANY of the workstations.
I checked the user source and it all looked OK. Was able to browse OUs
and see users and groups. I rebooted the DC that the user source was
pointing to and also rebooted the ZCM server. Still no luck. Was able
to browse user source using LDAP browser. Changed the user source to
another DC but still no go. Rebooted the ZCM server with it still
pointing to new DC and after it came up was able to log back in again.

I've fixed the problem but it doesn't fill me with confidence that it
should suddenly stop working like this. No changes were made overnight
to the DC or the ZCM server. Any ideas?