Not sure if you solved your problem, but I had the same issue.
I flashed my WHR-HP-G300N (that had a very old release of the Buffalo DD-WRT on it) with the latest version and my access points would not do anything on the wireless side. The status page said "unknown" for channel and nothing was seem on any of my devices. I tried the recommendations here - reseting and reloading the configuration to no avail. I didn't reload the older firmware because I really wanted the newer features. My problem, which may not be the same as yours was that I had selected "N/G Mixed Mode" and apparently the new firmware didn't like it. When I switched it to "Mixed" the wireless side selected and displayed a channel on the status page and I could see the SSID's on the devices. I am using these in a WDS bridge mode and broadcasting a second SSID on the one AP. Not sure if that matters, but the N/G Mixed apparently was my particular problem.
