Clean install (not upgraded) on '10 MacBook Air seems to join the 5GHz network only if it is advertising the SSID (i.e. closed network is not ticked in advanced settings on the AirPort Extreme). As soon as I check the 'closed network' box, only the 2.4GHz network can be found and joined. Note that I use a different wifi network name/SSID for 2.4GHz and 5Ghz - both are added to the MBA wireless networks profile list.
My (Late) 2007 Macbook Pro with an upgraded install to the developer preview of Lion has no problem seeing and connecting to my 5GHz 802.11n network from my dual band airport extreme.
My (Late) 2007 Macbook Pro with an upgraded install to the developer preview of Lion has no problem seeing and connecting to my 5GHz 802.11n network from my dual band airport extreme.
I clean-installed the Lion dev preview on a new iMac today. During the OS X setup process, I couldn't connect to my dual-band AirPort's 5GHz network (the network is "closed" / hidden SSID). But, after the setup process, I was able to connect to the same network through the Network prefpane.