I just tested this situation in a developer org - upon deactivation there was no warning/error message stating that the user was an active email service's context user, and the email service itself was not deactivated (at least according to the UI). However, upon sending an email from an authorized source, the
handleInboundEmail method of the
InboundEmailHandler-derived class was never executed. Interestingly enough, none of the Deactivated Email Service or Unauthenticated Sender actions seemed to fire, either. Upon reactivation of the context user, the service began to function normally.
I'll give this a shot in an Unlimited edition org and see if the results differ. (Update: results were exactly the same in the Unlimited edition of Salesforce)