Maemo 5 on iPhone? Not possible because of all the closed bits of code on Maemo 5 (cellular services daemon, phone dialer, virtual keyboard, messaging client, internet connectivity daemon etc etc).
MeeGo on the other hand could in theory be ported if you could get drivers for all the hardware (display, GPU, camera, WiFi, bluetooth, GPS, cellular radio etc). But given how many phones are out there that are possible targets for MeeGo or some other Linux platform that are far easier to work with than an iPhone (Nexus S for one), why would you even bother.