I have an early 2011 MBP and a current generation Time Capsule. I have set up the WiFi to be dual-band with the same name.
When I boot from cold, my MBP automatically connects to the 5Ghz band as you'd expect, but when it comes out of sleep it always connects to the slower 2.4Ghz network. If I switch off the WiFi and turn it back on again it does connect to the 5Ghz band. I've seen this problem on both Lion and Mountain Lion.
I suspect this is just a bug but can anyone can suggest a fix?