For perspective I want to state that the comment in the OP talking innovation is software (aka gameplay). On that note Sony has, without a doubt in my mind, made attempts to innovate. Eye of Judgement is a great example. That's a game I've never seen before anywhere. Unfortunately, the sales of EoJ were horrible and money was lost.
And I think that is the point of the OP. Microsoft and Sony have made a machine that has really good graphics, they marketed their machines based off of how good their graphics are, and they created an audience that demands great graphics. Great graphics make for large budget games. The larger the budget the more risk adverse a company (any company large or small) becomes because of the potential losses.
The wii has certain advantages over the 360 and ps3 for innovative software. Part because of the unique controller, part because the wii audience doesn't demand lush graphics (meaning a B team game (like RE:UC) won't be immediately dismissed), part because the budgets are smaller so if a game flops it's not quite as big a deal to the companies bottom line.
summary..
To say the 360 and ps3 are innovation killers is definitely not true. What is true is companies will be somewhat more risk adverse when developing for them compared to when they developing games for the wii.
