With 2.10NA and the "Channel Width" set to to auto 20/40, the router starts in the 40MHz mode and correctly switches to 20 MHz if there are other routers close by. However, it never switches back to the 40MHz mode when the interfering routers are removed. I thought I was having an issue with my setup, but a lot of people are experiencing this problem.
http://www.broadbandreports.com/forum/r25983100-DIR-655-Hw-Version-B1