This thread over on the Quarter To Three games forum discusses whether the PC was the “real winner” of the latest generation console wars. The thread author suggests that the PC will 'win' by virtue of having lots of console games converted to it at a later date, but I think the reality is a little more subtle. If there is something to be learned from the current spread of games then it's not necessarily the brute number of conversions to PC that matters – it's the money going into the really significant games. Publishers need to figure out how much they can spend on a game, and they'll spend more if it's likely to do well on a console as well as a PC.
Quake Wars has undoubtedly benefited from being converted to 360. The console format meant that the team had to factor bots into the game – a fact that gave them extra months to polish the game as a whole, and also means that instead of a pure multiplayer game we also now get bots to play with.
Bioshock is another example. There's little doubt that it would have been a masterpiece whatever the format – but more money would have been coughed up for the overall development costs because it's going to be on 360, and not just 'conversion costs' either, since these tend to disappear into the overall equation of running a studio anyway.
Perhaps the only “real winners” are the gamers, on whatever format they choose to play. As long as there's no monopoly, no dominating format, then publishers and developers will be compelled to deliver their games for everyone, however they choose to play. If a developer keeps their game to just one format, and invests accordingly, then the studios aren't going to be able to exploit those deep pockets to their fullest potential. Cross-platform profit margins, I salute you.