Last night during an electrical storm, a portion of our building lost power (the entire building isn't protected by a generator). Several client machines couldn't login to the domain when the power came back up. We were seeing a message that basically said the machine didn't have a trust account on the domain, yet AD said the machine was a member of the domain. We couldn't login with cached credentials, and local accounts failed too. We tried the restore procedure many people had suggested and that wouldn't work either. The solution ended up being simple. We physically disconnected the machines from the network - unplugged the network cable, and successfully logged in using cached AD credentials. The downside is that some users lost some data. Since none of our XP machines were effected, we're thinking Microsoft needs to address the issue since with Windows 7. It seems to be related to machines being in sleep mode and encountering a power failure.