TL;DR : I think the X-37 is nothing more spectacular than an upgradable spy satellite that can change it's orbit of inclination giving it the ability to spy on any part of the globe as quickly as possible.

Although the X-37B's mission is a closely guarded secret, many facts are public knowledge. I compiled a quick timeline a while back and seeing as it is in the news again, I am reviving it.

1998 : During the Clinton administration, Boeing test drops the X-40A, a test vehicle that would later be developed into the X-37. The government's own National museum of the US Air Force [freely states](http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id...) "The SMV program aimed at developing a new generation of small and reusable, highly maneuverable space vehicles for deploying satellites, surveillance, and logistics missions."

1999 : Public acknowledged start date of the X-37 project. Obvious similarities between the X-40A, launched a year earlier, and the X-37 show that work had already been started on the project, in one form or another.

2004 : The Bush administration decides to shake up nasa and outlines it's [Vision for Space Exploration](http://www.nasa.gov/pdf/55583main_vision_space_exploration2....). In it, they outline the future of spaceflight laying out the, later cancelled, renewed efforts to send manned missions to the moon and later mars. In it they talk about a new generation of launch vehicles. This document is the beginning of the end for the already aging shuttle program. In it, they spell out that the shuttle program's only continuing purpose would be to finish the international space station.

2004 : Also in President George's plan is the further classification of the X-37/X-40 projects as they are [moved from NASA to Darpa](http://www.space.com/337-nasa-transfers-37-project-darpa.htm...) where a tighter lid can be kept on the whole project.

2006 : First Drop test of the X-37

2009 : Barack Obama is elected POTUS

22Apr2010 : First launch of the first X-37, Mission USA-212, and lasted 225 days

5Mar2011 : A different X-37 is launched and lasted 469 days

11Dec2012 : Another X-37 is launched and is still in orbit at the time of writing this. It is possibly that it is the same ship as was used in the first launch. As of today it has been in orbit, 657 days. USA-240 Mission

8Oct2014 : [NASA Partners with X-37B Program for Use of Former Space Shuttle Hangars](http://www.nasa.gov/press/2014/october/nasa-partners-with-x-...)

13Oct2014 : [The X-37B is reportedly going to land in California Tommorrow](http://news.yahoo.com/secretive-x-37b-military-space-plane-c...)

My personal conclusions on all this lead me to believe the following however I highly recommend anyone with interest in the subject research it and make your own conclusions as I might have missed something:
Due to the long amount of time they have kept each X-37 in orbit and the apparent desire to keep one in orbit at any given time, I personally believe that the mission is ongoing and of perceived strategic importance to the united states. Given that the US government itself has described the X-37's predecessor as a "highly maneuverable space vehicle for deploying satellites, surveillance, and logistics missions" I believe this is exactly what it is doing. It is an orbital space plane with a next generation optics and surveillance package that has the added ability of being able to change it's orbital inclination in the event that a certain spot on the earth needs observation urgently. Changing your orbital inclination costs a lot of fuel so you still want to be able to deorbit to be able to refuel and have another ready to take it's place when you do this. This also gives your engineers at Boeing time to make improvements and upgrades on one bird while the other is flying keeping your optics and sensors at the cutting edge all the time.