The idea of a union for MMA fighters has been bandied about for a long time now, but it's not something that's very realistic at the moment. Much like pro wrestling, a union has never ever come close to being enacted, mostly because those at the top aren't usually very willing to give up what they have to help the greater good. Surprisingly, UFC president Dana White agreed with that notion when the topic came up in a recent ESPN SportsNation fan chat:

"I doubt it. The thing about fighting is, fighting is not a team sport, it's an individual sport. It's going to be tough to see a day with Silva or GSP is giving up big chunks of their money to guys who won't make two fights in the UFC. Different sports. But if it happens, it happens. I have to negotiate with somebody on the fight contracts."

The part he's leaving out is that he's the one that's the one pulling the strings. As long as he keeps a few guys very rich and very happy, it's not something he's going to have to worry about.