With latest update from Maemo and latest software and OpenVPN configs you'll be able to connect to OpenVPN.
!Update 02.04.2010. Works only under WiFi connection.
1. Go to http://maemo.org/downloads/product/Maemo5/openvpn/ and install OpenVPN v2.1~rc20-3maemo3
This will install only OpenVPN software, if you are familiar with Linux you can use it from x-terminal line without GUI.
2. Install OpenVPN GUI: to install OpenVPN Applet v0.5.3-1 go to http://maemo.org/downloads/product/Maemo5/openvpn-applet/ and press install.
Nokia N900 OpenVPN GUI
3. Download OpenVPN config files from here. Extract turnkeyvpn.zip archive to "hideipvpn" folder on your PC/Mac and upload turnkeyvpn folder to your N900. Or download directly to your N900 if you have unzip software.
4. Open OpenVPN GUI and press: Manage connections -> New -> Configuration File
5. Find turnkeyvpn folder which you uploaded in step 3 and open configuration file for example "Turnkeyvpn-UK.ovpn"
This will complete all needed fields.
6. Press Import button.
This will create new OpenVPN connection to UK servers.
7. You can now connect to the VPN