AA and UA do status challenges - They are probably the two biggest carriers, but I'd look at their forums (as well as others - US, etc.) to get an idea of which carrier's routing works best for you.

Threads that may help:
http://www.flyertalk.com/forum/delta...ve-thread.html
http://www.flyertalk.com/forum/delta...turn-coat.html
http://www.flyertalk.com/forum/delta...tching-ua.html


AA: http://www.flyertalk.com/forum/ameri...l#post11822023
UA: http://www.flyertalk.com/forum/unite...ed-2010-a.html