I've now done a lot more experimenting. First, I dropped DD-WRT and went back to the D-Link firmware, though upgraded to 2.05NA. With that, my computers did see the 5GHz band (both with the usual network utilities, and, with more details, inSSIDer), but were unable to connect to it. I then realized that I had a Mac with an 802.11n-compliant card, and lo and behold, it was able to associate right away.
I'm not sure what the problem could be with the Intel cards, which I can't remove from the laptops as they are internal. Will look into this as a probable cause for these issues, though I don't know what I can do about it. If that's the only problem, fixing it might fix everything--though since I've heard other reports of flakiness on the 5GHz band, maybe not. (Was that just with earlier firmware? How stable is it with the latest release?)
Thanks for the suggestions.