While I was finishing up the "ESBSimpleSamples" project, I came across this strange error. On submitting a message to the ProcessItinerary Web service I got a 503 Service Unavailable error, and the event log showed the app pool had been shut down, with The identity of application pool, 'ESBAppPool' is invalid and error code 80070532.
A quick Google suggested it was an issue with the BizTalk isolated account. I hadn't changed it at all and the service was fine a couple of days ago, but I checked the BTSISOSVC account. It was flagged as "User must change password at next logon" which seemed odd. Removed that and set it to never expire, restarted IIS and it all started working again. Odd.