Yes, I'd likely buy it, but there would be a greater chance of device-migration before contract end.
I have an iPhone 4 and I have already started looking at other smartphones. I actually bought (but returned) a Droid RAZR because I am bored with the iPhone style (both in screen size, wanting of 4G, and a different interface). The iPhone 4s (and honestly even the iphone 4) processor, screen resolution, and camera are all satisfactory, so if it does look the same, I dont really know what else they would improve on to make it desirable enough for a 2 year contract.
While I dont think the software interface will change much, refreshing the physical design a bit would at least make people feel like they have something new... I would think for people who arent tech-heads, if the iPhone5 looks the same, there would be little reason to update to it.