My first thought on AT&T's acquisition of T-Mobile USA was that it's by no means guaranteed that the competition authorities will allow something that gives one operator 43% market share. Initially it looks to be handing too much power to one player. However, if US authorities choose to look overseas they'll find that such a thing is the norm. Eighteen months ago, when T-Mobile and Orange (which co-incidentally also gave them a market share of 43%) were merging their UK assets I examined what was a normal market share for a #1 operator in Western Europe. When that merger was announced, the UK was the only market in Western Europe where there was no single operator with a market share of >35%. In 11 of the 15 other major Western European there was an operator with 43% market share or greater. So this merger puts the US on a par with the norm in Europe.
However, there are some differences. Firstly the scale. European markets are much smaller and most operators operate across multiple countries. So, for instance, Vodafone, T-Mobile or Telefonica will make some handset purchasing decisions on a regional basis. To some extent what is important in Europe is regional market share, and this is much more fragmented.
Secondly there is the issue of technology. While AT&T/TMO has a 43% market share in mobile it has ~85% market share for GSM, and thus a de facto monopoly for the purchase of handsets and network equipment*. That's not what the general public would consider to be the issue for consideration by competition authorities, but it's definitely a competitive issue.
Thirdly there is the question of whether it will spark further mergers. A T-Mobile/AT&T tie up creates a GSM powerhouse with 130 million customers. It is likely that the CDMA operators will consider doing the same to give themselves 150m subs and a nearly 50% market share, thus effectively creating a duopoly for services and another monopoly for the purchase of handsets and infrastructure.
* Once everyone starts rolling out LTE equipment the network element disappears, although handsets is still an issue as they'll need to be multi-mode, either CDMA/LTE or GSM/W-CDMA/LTE.