After upgrading to Yosemite my mac refused to see some of the wireless networks I was using before.
Those networks used the 802.1x (and PEAP).
The funny thing was that the machine was hooking on the network, the wifi connection was there but it was unable o reache the Internet. After googling a bit, I found a solution on he Apple support community.
So I moved /Library/Preferences/SystemConfiguration to a backup folder (sudo renamed it) and then restart the machine. Of course same of my settings were gone, but at this point the Mac happily joined the Internet bandwaggon.