Several times a day I'll close my MacBook Pro laptop with the power still on, then later I'll open it again, unlock the screen with my admin password and wait for it to reconnect wirelessly to the Internet. (My power settings are set so that it disconnects after being closed for a certain period of time.) 9 times out of 10 it's successful in reconnecting, but every once in awhile it fails to reconnect. Usually when that happens, all I have to do is close and reopen the laptop, unlock the screen again, wait for it to try again and then it works, but that's so annoying. Why does it do that? I should also mention that I've had the computer for 3 years and it's only done this in like the last year. It's possible that it started happening when I upgraded my wireless router to an 802.11n.
I had the same issue with my previous MacBook Pro (late 2006). One solution was to turn the wifi off, then to turn it on again (faster than closing & opening, you can do it through the menu bar icon).
I guess it's due to some timeout in detecting the surround networks and it may happen especially if there are many of them.
There is probably no fix to that and won't be any in the future.
One of the solution would be to change the time of detection, but as far as I know, there's no way to do that.