All these clever hacks to prevent IP address clashes - or at least stop them from being a problem.... I've done similar things myself on other devices - which need to connect to a wired and wireless network at the same time, with each network having it's own independent DHCP.

So what does the N900 do when you're on a WLAN, 3G, or Bluetooth PAN at the same time?

That can clash too. Easily: I've had 192.168.x.y addresses assigned when connected over 3G on other phones. The same ranges are often used for WLANs, and on PANs (I mean for local file transfer among Bluetooth devices, not providing an uplink).

That must have been solved somehow. How's it done, and why can't the MMS gateway use the same method?

Please don't tell me you can only have one of WLAN, 3G or PAN running at once!