I use tunnelblick and OpenVPN on the mac, but when I choose to disconnect from the VPN I lose all network connectivity and have to reboot to get back online.
Is there a way to get a new IP or something, without having to reboot?
You could enter your network settings, find your physical network interface, and use the release/renew buttons (in that order).
This would cause your interface to attempt to request configuration again from your router.
My guess is that the issue is that your routing table isn't getting it's default gateway reset to the IP of your router after OpenVPN tears down it's interface.
To see if a better solution (one that involves less work) could be found, could you copy the output of
Try disconnecting, then reconnecting the network. Either unplug Ethernet and then plug it back in, or turn Airport off and then back on.