Although it has not happened recently, I think it is likely that the Justice Department would attempt to block a UA/DL tie-up for example. I don't think it's clear what the reaction would be to a UA/JB or a DL/AS.
It is not enough for the DOJ to show that the combined airline would own a particular percentage of a market, they must further prove that this increase in market share, would give them a substantial amount of pricing power that they would not have had otherwise. XM & Sirrius were allowed to merge because although the combined company would own 100% of the satellite radio market, competition from other forms of radio and entertainment in general would not give the company any substantial increase in pricing power.
This is why there is always so much talk about the overlap in the routes involving the slotted airports (DCA, LGA) where the barriers to entry for competitors are high. The barriers to entry for competition to come into an airport like SEA is fairly minimal. If Delta-Alaskan Airways tried to jack the prices in SEA upon merger, it could be argued that price increases would be kept in check by United having the potential to up-gauge there Seattle flights for example.
Again, I'm not arguing that a merger would happen, I'm just saying that DOJ blocking the next airline merger is not as black & white as some are claiming.