Infinity Ward is a pretty bright bunch of devs. CoD6 will certainly be improved over CoD4, although I dunno by how much.
CoD6 is hurt by the fact that its not an exclusive, however. I think any dev who favors the 360 in cross-platform development is going to find that, in the end, they are unable to outperform the games that do embrace parallel processing, on either platform. By focusing on the PS3 first, you're forced to think in terms of parallel, asynchronous threads, and you're going to unlock the most power in both architectures.
Devs have gotten away, on the 360 especially, with "use 1 core most of the time, or 3/6 when its absolutely necessary" for too long. You'd be shocked to know how many, and which, games have only utilized the PPU, or just one core on the Xenon.
