An iPhone 3GS
teardown with detailed chip analysis shows that the 3GS contains a different Broadcom part than the 3rd generation iPod touch: the BCM4325. According to
Broadcom's product page, this part truly only supports 802.11a/b/g (no n), in addition to Bluetooth and FM (receiver only).
So, yes, the 3GS contains a chip which could be used to receive (but not transmit) FM radio. That being said, the chip by itself is useless unless the correct antenna is hooked up. So far I havn't read about any evidence to suggest the presence of such an antenna connection.