Darren
I am not clear what you are trying to accomplish but I do not think that you want to do it this way. If you configure FastEthernet0 with no ip address, then the router will not send IP traffic over that interface.
If you are trying to conserve address space I have seen routers in situations similar to this which configure serial0 with ip unnumbered fastethernet0, and put the IP address on the FastEthernet. This does require some coordination with the router on the other end of the T1.
If that is not the solution that you are looking for, then perhaps you can clarify your environment and what you are trying to accomplish.
HTH
Rick
HTH
Rick