It been what I've been told. I think it might be related to GFX core and whatnot else. Sure the *basic* CPU opcode should be portable, but the SoC differs too much - aiui.
If we could guarantee that fremantle incl all blobs can work on a OMAP4, all the better! Nikolaus mentioned that we might even have the needed additional 0.8mm headroom in N900 case that were missing in GTA02 case, so we *could* go OMAP4. Then OTOH I know about SiErr in all OMAP4 that are declared "WONTFIX in OMAP4" by TI. So maybe we do not even want to go for OMAP4.