You mean ASICMiner, Avalon and BFL? I think they try each on their own to make their profit. As the bitcoin is quite stable in the last time I think there isn't anyone manipulating the market yet. But I see the 20% of the networkhashrate of ASICMiner as a big problem. There are pools that have a higher hashrate now, but in a few months when most of the GPU miners will have stopped bitcoin mining and ASICMiner bought even more ASICS, then they might be up to 40%. And from 40% it isn't a long way to 51%...

If they realy would want to merge I think nobody could stop them. But I hope that they are smart enough and know that they would kill the bitcoin with that.