A number of users have encountered problems connecting or staying connected to wireless networks using Mountain Lion. This combination of steps from the folks at 
seems to work like a charm. Click here to check it out on their page.
Tag Archives: wireless
Setting up a wireless connection – Updated
You can see the latest wireless setup informaiton here at campus ITS:
http://www.unmc.edu/its/wireless.htm
Update: On that same page, there is now a link to setup the wireless connection automatically which has worked for a number of people 🙂