I just upgraded to OSX Mountain Lion and had some trouble this morning getting my OpenVPN connection (using Tunnelblick) to work. It would connect, but the DNS resolving was unstable. I managed to get it to work on the latest stable by changing the DNS-server setting to ‘set nameserver 3.0b10’ but then internet traffic would not work. The best solution for me at the moment is to upgrade to Beta release
After upgrading, everything works as expected!
Update: see this blog I wrote as well (experience after one week)
Update 2: TunnelBlick 3.3 beta 18 has just been released and the release notes state it has fixed more Mountain Lion bugs.