If the chipset doesn't do much of the decoding and passes on the 5 MHz of bandwidth to some post processing application, you, in theory, could strip out the 1.25 MHz bandwidth of your choosing however, I'm certain that the chipset handles more than just the filtering of the out of band signals and once the correlation and decoding algorithms take hold, getting any other information out is next to impossible. I'm being deliberately simplistic.
But if you were going to attempt to make that chip do something that it might not even be capable of, you'd need access to schematics and periphery so that you could even attempt to understand how you might go about that, or you'd need about 2 solid weeks with an ohm meter and a damn good documenting strategy. But my guess is that the Infineon chip is so specific in order to curb power requirements that it is most likely impossible to make the iPhone chipset do anything other than WCDMA or GSM protocols.
We're getting at the same point. That what the OP is suggesting is most likely impossible, without significant modification to the phone or some great inside knowledge.
My point is that it's not impossible for a piece of hardware to do be capable of handling 3G, GSM, and/or CDMA, but the likelihood of finding that hardware in a mobile application is very slim since you most likely sacrifice power for the flexibility.
