Yes the PS3 and 360 are quite similar in power it would seem so far (though which is truly more capable of having it's power tapped into by developers will probably be up in the air for a few years), however they're very different system architecture's. Just because they're similar in power doesn't mean you'll be able to simply port a game engine designed from the ground up to utilize the PS3 hardware over to the 360. Sure, it'd most likely be possible with some tweaking, but the PS3 does feature essentially parallel processing with a different graphics card, etc. The likelihood that you could just take the game and have it instantly running on both is slim. Just how much work would be involved in optimizing it for both systems is anyone's guess though. Only the developers would really have that answer.