The YTD sales were my "evidence" - YTD the 360 looks to be doing the worst compared to both the Wii and PS3. Of course, I have no hard evidence that a price drop of x would amount to a sales increase of y (eg, a $100USD would make YTD sales 2x their actual number) but the fact is the 360 is the only one that isn't gaining a lot of momentum. It has the games, it has the game sales, and it has a huge online user base.
The only significant thing that I can see would get more people buying the 360 than those that have already jumped in is a big price drop.... purely speculation of course, but that's what all of this talk is, at the end of the day!
Realistically though, after sitting for two and a half years at roughly the same price ($50 isn't too much of a drop!), don't you think more people would say "now's the time to pick one up" if the price dropped?