Update 3/19/14: Done! You can now pass under MLK on the trail. The trail extensions described below are underway, but currently you can't really go much of anywhere after using the tunnel.
I've been trying to find a good time to photograph the tunnel progress, and a recent trip to the Farmer's Co-op for a snow shovel and local eggs provided a perfect opportunity. The tunnel under MLK will help connect much of south Fayetteville to the Frisco Trail and greater Razorback Greenway. City plans for the trail show it forking south of the tunnel with one branch connecting to the Walker Park trails and the other paralleling MLK to pull in traffic from all the new large apartment complexes. This will allow residents of those complexes a safe way to get to campus and Dickson St. by bike or by foot instead of having to battle traffic and fight for parking.