There is a lot of different ways of looking at it. As others have said, the 360 saved the PS3 by giving developers a large enough user base to develop HD games for. And of course by that argument, the PS3 has saved the 360 in the same way.

If the Wii never existed, I still think the 360 vs PS3 battle would have played out in a very similar vein to how it has so far. If the PS3 didn't exist, most HD gamers would have gravitated over to the 360 so it'd be still Wii vs 360, and same for if 360 never existed. Although I will say that given the PS3's entry price and Sony's PR blunders during that launch period, the 360 would have competed better against the Wii than the PS3.

really doubt 360 or ps3 would have gotten much more sales without wii. i guess at least 50-70% of the wii buyers would not have bought another console without wii. the rest would probably split between ps2, 360 and ps3 (i guess at the launch time of wii people might have bougt more ps2 without wii too).
sales for hd consoles might have been 10-15% higher without wii, but i doubt there would have been that much changes to the distributions between ps3 and 360.

Microsoft saved the PS3. Microsoft have funded development of a lot of HD games this generation. Without an injection of $$$ from Microsoft paying for timed exclusives and multi-platform games, the gaming industry would be in much worse shape. Microsoft has established itself in the game industry through its continuous cash injections to struggling developers buying DLC and multi-platform games.

So the Wii sales have slowed down the 360 sales. Big deal. There are three wars within a console war this generation - a three way consoles sales war which the Wii is winning, the HD format war which Blu-Ray in PS3 won and now the battle for second place in the console war which does not really matter.