I have a MacBook Pro (2.4 GHz Core 2 Duo, running OS X 10.6.8) that does not reconnect to its stored WiFi network after it restarts or goes to sleep. To reconnect, I need to go into System Preferences and choose a network in Airport settings. Then it happily reconnects and stays connected.
I saw this question, and tried to delete and re-add my preferred network (I only have one). I went into Advanced, deleted the network, turned Airport off and on, and chose my network from the dropdown. It knew the password already though.
I was going to delete/move the preferences file containing network/WiFi settings, but I can't find it. Would this be helpful? If so, where is that file stored? I've already looked through ~/Library/Preferences and /Library/Preferences to no avail.