My 11" MacBook Air never remembers wifi...

The problem is in your settings. U must check - Ask to join new networks box in Settings - Network - WiFi

In Advanced option there is another box to check - Remember networks ...

check it !

On my MBA 11 everything works OK and remember passwords.

 

mine forgets maybe 20% of the time. its weird.

 

Mine has done this since day one. And I think quite a few people had this problem also.

I don't have to enter the password or anything, I just have to go onto my WiFi and re-select the network again. Sometimes I'll have to do it twice as the first time I get a "Connection timeout" error.

 

Mine remembers the network when plugged into power but if the MacBook has been sleeping unplugged it is almost guaranteed to have forgotten the network. Mildly annoying.

 

This has been an intermittent issue in OS X 10.7. Reportedly, 10.7.3 will fix this somewhat.

 

After waking from sleep, mine sometimes has amnesia and connects to a btopenzone spot

 

This is sadly a known problem. Turn off bluetooth and it will stop doing that.