There was an old thread about openvpn configuration for the Nook classic here, but I recently tried the version for the Nook Color (
openvpn instructions and troubleshooting here) and can report that I have had success using with Witopia openVPN service (a pay service).
I usually use my EVO phone as a hotspot (even when there is public wifi available) but some places these days are so saturated with strong on-location wifi that the weak EVO hotspot signal isn't enough. With my PC, I can just plug in my EVO and share connection over USB, but that's not an option for the Nook. So being able to use VPN with the public wifi is something I wanted.
Configuration is not trivial, but I did want to report success. I used a terminal emulator to issue the shell commands on the device instead of adb, but otherwise I followed the instructions given, plus a few troubleshooting hints later in the thread. It also doesn't seem to handle reconnect from sleep well -- gets stuck in a wait state upon re-establishing vpn connection, but restarting the connection resolves it.