Over the past 2 years I have regularly watched fares to Memphis and Nashville, they are usually in the $400 range which is very high on a miles travelled basis. Very rarely do they drop. About two months ago fares to Memphis, served by both CO and NW, dropped to about $250.

My last two trips to Memphis I used miles, the only time I have used miles in the last 5 years for domestic trips. The IAH to TYS market is served non-stop only by CO as far I know and therefore unlikely to drop in my opinion. AirTran may run decent fares for a connecting flight.