For Japan the roles are the other way around, the PS3 has a 3 to 1 install base advantage. But the sales gap for many simultaneously released PS3/360 games are beyond a 3 to 1 advantage.
For example recent games like RE5, Tomb Raider: UW, Midnight Club: LA performed rather 4:1 to the PS3's advantage.
Really I don't see the point why some people are attacking Sony and are defending Microsoft's PR...
Multi-platform games perform no worse than on the 360, actually in general they seem to perform slightly better than one would expect based on install base. Especially from an amount of software bought per user per month of ownership perspective.