I like that answer and wish it were the case. Sadly, the Bluetooth on the 3GS is off. To remove all doubt, I turned the phone completely off. Also made sure the iPad's Bluetooth was off for good measure.
Still, the iPhone sees nothing anywhere it wants to pair with. I am left with
Settings > General > Bluetooth: On : Devices=Searching : Now Discoverable
The "Searching" timewheel is keen to spin to eternity.
If I activate BT on the MBP and run Bluetooth Setup Assistant, it sees the 4S and identiies it as a smartphone. Upon selecting "Continue," a code is generated on the MBP and received by the 4S. Okaying the code on the 4S gets a timewheel spinning. Eventually a "Pairing was unsuccessful" message appears on the iPhone.