Thanks. I actually just made some progress...
I had previously added a syslog server on my network in an attempt to capture what might be happening when my network was dropping.
I hadnt had much luck pin-pointing anything of interest until today. I found this happening right when all my devices were losing connecting to my network -
2012-10-31 15:45:52 notice airport-extreme local0 80211 Switching to channel 60 2012-10-31 15:45:52 notice airport-extreme local0 80211 Radar detected on channel 132 2012-10-31 15:45:52 notice -airport-extreme local0 80211 Switching to channel 60 2012-10-31 15:45:52 notice airport-extreme local0 80211 Radar detected on channel 132
It appears that the required DFS controls for these upper channels (in UK at least) are controlling things on their own (as designed to).
I have turned my 5Ghz network to a fixed lower channel so I shouldn't get these network drops.
If you don't hear from me, all went well!
Here's what I did to fix the issue with MacBook Air 2012 and AirPort Extreme:
1. Reinstalled OS X (optional, not sure now it was really necessary)
2. Reset SMC and NVRAM
3. Reset AirPort Extreme with latest firmware available (7.6.1 as of now)
4. Name your network starting with some symbol like $ or # or whatever (don't ask why - tech details). Should lookd like $Airport_Network_Name or #John_Appleseed_Wifi or whatever.
5. Set the correct country (or the closest to you) in AirPort utility settings for AirPort Extreme (was Russia in my case since I'am in Ukraine)
6. MacBook didn't see the 5 GHz network at all even if I entered the name, security and password manually. I had to choose lowest of the available channels for the 5 GHz network (36 in my case)
7. Scanned surrounding WiFi networks via WiFi Scanner utility from Mac App Store (https://itunes.apple.com/ru/app/wifi-scanner/id411680127?mt=12) to see which channels neighbours are using and chose a free one for 2,4 GHz network. For example, surrounding networks use channels 3, 7 and 8. I chose 2 or 5 or 9 etc...(did not create separate 2,4 and 5 GHz networks!)
As a result MacBook Air 2012, MacBook Airs 2011, MacBook Pro 2011 and iPhone 4S are now all compatible and work the right way with AirPort Extreme, automatically connecting to 5 GHz (Mac) and 2,4 GHz (iPhone) without any issues.
Hope this helps.