A bad SFP port on a router can be tricky. We just had a similar situation here. You go into all kinds of detail on the port setup and the routing and then it all comes back to being a layer 1 issue. Aaarrrrrr! Anyway, let us know how this turns out.

Have you checked that the GBIC you are using in the 3750 is good?Also, the port itself could be bad in the 3750. Set up a test link with that port and another gbic and see if you get a link to a test port in another switch.