This question has an answer, but it only works on the old iPhone and iPod touch.
On the iPhone 3G S, it gives me "00:00:00:00:00:70", which is incorrect.
Stupid me: I had Wi-Fi turned off. Actually, even if Wi-Fi is turned on, but is not currently connected to a network, it doesn't show up!
The Wi-Fi interface appears to be called en0, while pdp_ip0 (the interface for the MAC I mentioned in the question) is the cellular network.