Since this has been pretty heavy in the rumor mill lately. The way I see it takes three things to make this happen, with the first two being a no-brainer....
1) UFC. From what Dana White says he's all in. This fight makes tons of money and generates lots of interest.
2) Anderson Silva. He's the one doing the calling out, and it appears he's content to just hang out until a big payday fight comes along. Why fight a guy like Weidman who most people wouldn't recognize if he was in line next to you at the grocery store when maybe wait a few more months and make 5x the money with a fight in a smaller weight class with one of the most recognizable names in MMA?
3) St Pierre. The x-factor, I've not read much about his thoughts on taking this fight but sounds like he's at least considered it if he's laying out terms like it being at 180. He's also the one who could probably have well received matches going forward with plenty of young talent at 170. I figure bottom line he doesn't have as much to lose since he's the smaller weight class guy, if he loses then he was expected to and his title won't be on the line.
Regarding the fight, I don't think it is as lopsided as some think.
For one the size difference won't be that huge. They'll both cut to make 180, but Silva will have to do a lot more of a non-water cut before the fight. If Silva hits the scale at 180 lbs he's likely walking into the ring at right around or under 200, and I think St Pierre will be at 195. Silva is much taller but St Pierre has a very long reach for a WW, the difference in reach would only be about an inch and a half.
Also I think St Pierre can take him down. Sonnen took Silva down, Henderson took Silva down, hell Travis Lutter took Silva down. GSP's takedown ability are unmatched in MMA, I don't think a 10 lb difference in weight is going to mean suddenly the guy who can take anyone down at will suddenly cannot throw a double-leg onto Silva who's shown a weakness at wrestling. I'm don't think an Anderson Silva who made 180 is going to be any smaller than Thiago Alves, who is far more and muscular and lower center of gravity. GSP's striking game is also very underrated, he's no Anderson Silva but he also isn't some plodding wrestler with no other dimensions to his game.
All of that said, I'd put my money on Silva TKO 2nd round.
But I do think it'll be a lot more of an interesting fight than some people think.
Also = Condit isn't relevant here, I believe GSP will walk thru him. I like watching Carlos Condit fight but doesn't do well with strong top control guys (see Rory McDonald despite the win) and no matter what ring rust or concerns about explosiveness due to injury I don't see anywhere that Condit poses a challenge to GSP. I don't think the Condit fight has any bearing on St Pierre being available and willing to fight Silva. Sorry Carlos, is what it is.