There *is* a domestic gaming market in Japan, something we often forget as it's been stagnant for some time... but it is still quite large. No doubt the IPs Japanese devs are used to exporting to the West anyway are going to be titles that could readily find their way onto the 360. But any titles that would launch in Japan first (such as the Final Fantasies) or have an outright Japanese focus period are going to have the PS3 in mind during development, because it would be hard to imagine at this point the 360 gaining the momentum necessary on that island nation to warrant a primary development focus for devs making games targeted towards the domestic market.
This is important, because as long as engine development continues apace for the Cell/RSX seperate from what's going on said Western-targeted titles, that knowledge will trickle down through the rest of the J-dev community, and ultimately should result (well I'd hope) in some games that whether multi-platform or not, make good use of the Cell.