I've had two unnsigned checks returned by the corporate payee but none from a bank; I have sent a half dozen unsigned checks in my life time (by accident).

In our negotiable instruments law class our instructed said a bank would not cash an unsigned check; there was a class debate. I was chosen to send an unsigned check (to a colleague) that was ultimated forwarded to my bank in my town of 600 where I was well known (we'll not get into my character reputation) and the check was cashed.

So I must agree with Mike--big banks don't know your signature.