How to fix MacBook, not reconnecting to wireless network on wakeup?
Every time MacBook Air (2010) waked-up from sleep it would ask to choose network to re-connect and then took about 20 sec to actually connect. same if I turn wifi off and then on again.

solution that worked for me was: system preferences -> network -> SET SERVICE ORDER.." (in sub-menu next to "+"/"-" (create/remove service) at the bottom of  services list on the left).

DRAG WIFI TO THE TOP in Service order list -> "OK" -> "APPLY"

After that I am back to no "choose network to connect" pup-up's and wifi connects in 3 seconds. Hope that helps.