Wifi connection lost after sleep in mountain lion
tried deleting all known networks and wifi services and reconnecting, but there is no improvement.
MacBook Pro, OS X Mountain Lion, late 2011 13''
I was having the same problem after upgrading to mountain lion.
What i did was in Finder went to /Library/Preferences/SystemConfiguration/ and deleted all the networking plists (refer screen shot). Then restarted the Mac and in Network Preferences reconnected to my WLAN.
That solved the problem.