iPhone 4 Teardown
From looking at the Broadcom site about that part it appears to be a utility chip that they rolled out to replace older parts (probably the ones in previous iPhones etc) The chip supports both 2.4 and 5ghz for full wifi a/b/g/n and includes the FM transmit and receive. Steve Jobs said in the Keynote that iPhone 4 supports wifi n at 2.4ghz only so FM Transmit won't be the only thing available on chip not in use.
However the main feature Apple probably looked at was the improved battery life and reduced 65nm process. The chip appears to serve Apple's engineering conditions and be not utilising some features they can reduce the draw on the power.
I don't think we will see FM Tx from this new device as I don't think we will see 5ghz wifi n (or wifi a). If they had an application of the tech ready to go Steve would have shown it off, as they didn't show it I don't think it will be activated later. However if they find a "compelling" reason to activate it we should see appearances in the next year and the hardware *could* support it.