Had the same problem with my mid 2009 Macbook Pro 13 inch running Mountain Lion. Temporarily (appears to have worked, issue may return in a few days, hopefully not) fixed it by doing this:

1. Make sure "restart programs" when logging in is NOT checked. See here: http://macnerd-presents.blogspot.com/201...

2. Rebooted and held down "command + s" to go into single user mode and run /sbin/fsck -fy and if you see "disk modified" at the end, run again until you see "your disk is ok" For more info see here: http://support.apple.com/kb/ts1417

3. Reboot and log in and see if the wifi is showing ok. If not, go into System Preferences and Network and add a new wifi connection using the default naming conventions. See here for more info: http://www.chriswrites.com/2012/05/how-t...

Good Luck.

Carlos.