(Regarding PS3 only) It would be nice from a Gamers perspective. At best, you can play a popular game without investing in an X Box, at worst you just don't buy it. For a consumer, there's no reason not to. For Microsoft, they would make a little bit of money from sales, and then lose a major incentive for buying a 360 in the first place. It doesn't make business sense. I don't think there's any more elaboration to be done. It be fine, but it won't happen.
(Regarding PC) Eh. I guess there's no reason not to. Assuming porting the game isn't too crazy
expensive, there's probably enough PC gamers out there who are kinda curious and will pick it up for 20 bucks. Id probably give it a shot on PC if it were on sale for $10. And PC is kinda outside of the console competition, so it makes business sense. If you do, fine, and I suppose they just might.