Ran into this one again today:
The view 'Logon' or its master could not be found. The following locations were searched:
of course all the files were present because it was working 5 minutes ago (and the 2 weeks or so before that).
I had been using the ASP.Net MVC source project to debug a few things, but reverted to the GAC dll before checking in so that the other devs didn't end up with some phantom project reference. Hence I thought I had not reverted my web.config (and ~/Views/web.config) changes properly.
Anyway half an hour later and a lot of head scratching later I thought I'd see if setting the System.Web.MVC reference to "Copy Local =True" would make any difference
End of story.