I'm running Lion Server on a Mac Mini. Recently I was trying to configure an attached drive for Time Machine sharing and realized I could no longer log in. A bit of research revealed what I consider to be a bug in Lion. Visiting the Apple Support pages reveals a quick fix.
Apparently in "some situations" servermgrd may not start automatically, which prevents login to the server application. Yeah, just a minor bug. Seems like Apple could just include a simple cron job to check for the service and start it automatically if it's not running. Would have saved me a few minutes of panic, thinking I had completely forgot my server credentials. At least there's no automatic lock-out for more than 10 failed login attempts.