Multiplatform games don't necessarily need to look the same on all platforms, or even have 100% the same features across all different spec platforms.
This gen already has shown a more powerful platform can have more features and have better visuals etc... witness some games a year or two ago when multiplatform games on xbox had online component, had higher resolution textures, better framerate... Of course there are many mp games that are essentially identical, but most do have that 1.5x improvement. Then again there have been games where the game for more powerful platform have been inferior, it's up to the developer really.
During 2006 we'll certainly see most mp games that were coded with xb360 in mind, and ported over to pS3 with minimal or none improvements, or even downgrades. But I think it's possible the devs could switch over to PS3 as their primary dev platform and then downgrade the games over to xbox360.
But the main game structre in mp games will of course be done so that the game will be possible to port over to the lowest specced machine that has a strong market for those games to make them viable to port over.
After all, the next gen consoles area ll the same gen, so there won't be hugely massive differences in the technology behind the games. It won't be like porting a game from PS2 to DS
xb360 as becoming the "defacto dev platform", I really don't udersand that point of view. A "defacto platform" imo is the platform that devs expect to reap the most money out of game sales, that sells the game most, not the platform by which they spec their games to run at minimum.
Personally, I haven't bought that many of those "multiplatform games", I find them often too generic and not that interesting, and that there are often better alternatives within the same genre that are exclusives.
Like the Need For Speed series... or most of EA games or any "real sports" games
The big games buying public does seem to disagree though