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The iPhone 3GS's and iPod touch G3's Wifi/Bluetooth chip triples as an FM transmitter.

(Do you actually mean transmitter or did you mean receiver?)
Well the other thing to remember is that the 4.0 preview was really only focused on what it gave developers, because that is the only reason to show the software early, things like FM reception are not something developers need to know, so that would be something apple would hide and then tout as a feature when announcing the 4th gen iPhone. And as we have seen in the past, even if a device supports it, (The 3GS) that wont necessarily mean that it will get it. (Like battery percentage on 3G/2G for example).
 
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