Someone who goes against the establishment ..... someone who opposes or has a different point of view to conventional matters........... so basically anyone who is a "rebel" can be a Maverick ..... Go el Zorro!
I am pretty sure there are lots of people who don't really understand what Maverick means but hey... somehow they understand even more complex matters as far as the surge in Irak or the economy .... Go Rep party! Demystifying concepts and making the lowest common denominator somehow feel smart!
Links:
http://en.wikipedia.org/wiki/Maverick
http://www.thefreedictionary.com/maverick