The earnings call covered future 747 retirements ---
2 are being retired in 2015 but the rest of the fleet is now planned to fly until 2020. When they get to 2020, they'll reevaluate, as heavy checks will be due at that time.
From the earnings call, it sounds like the 2 being retired had reliability issues.

From the call:
Jeffrey Dastin - Thomson Reuters
How does the 747 fits into United’s fleet --planned fleet, retirement plan in 2015 and how do you only considered retiring a significant portion of 747s going forward?
John Rainey - CFO
The 747 is something that we do intend to keep for a few more years we have a couple coming out of our fleet in the near future but some of these we’ve made some improvements to the operating reliability of the aircraft and we could expect to keep them for another few years. They have another sort of big maintenance events in the 2020 time frame that that will be another decision point for us whether we want to extend them further at that point or go ahead and retire them.


If you look at United's cargo revenue, it increased 15.8% year over year. A lot of that's probably due to the Long Beach dock workers' strike, but is probably a factor in deciding to extend the life of the 747. With fuel prices down (after hedge losses/writedowns) more than 30%, the profitability numbers on the 747 are probably looking pretty good.