There are enough articles which describe how a VPN works, so I describe it very short.

• You need a server which has a VPN (OpenVPN) daemon running and is accessible for all clients.
• Your client (N900) connects to this server.
• Your PC, from which you want to connect to the client, connects to the server. (Actually your PC can also be the VPN-Server, then this step is obsolete)
• The server assigns every connected client an itself a private IP and establishes a Virtual Private Network (VPN) between all the connected clients
• Within this VPN all clients can communicate with each other, regardless how they are hooked up to the Internet.

There might be firewalls which prohibit a VPN connection.

OpenVPN is available for Maemo:
http://maemo.org/downloads/product/Maemo5/openvpn
http://maemo.org/downloads/product/M...openvpn-applet

How to generate keys and set up your configuration:
http://openvpn.net/index.php/open-so...howto.html#pki

Please get some basic knowledge about OpenVPN and try to create your own client/server config based on the example here:
http://openvpn.net/index.php/open-so...to.html#server
http://openvpn.net/index.php/open-so...to.html#client

If you do not have access to a own Server, here's a list of VPN providers (make sure they provide OpenVPN):
http://filesharefreak.com/2008/10/18...ice-providers/

If you own a Fritz!Box (With Freetz Mod) or a router which can provide OpenVPN service you can use it in connection with a dynamic DNS service such as http://dyndns.com (your router should of course support the dynamic DNS service) to work around the fact, that most ISPs don't assign static IPs.

Another home brew solution would be a Server behind the router which provides the OpenVPN service and configure the router to send all the traffic on the VPN Port directly to the Server. Again, dynamic DNS service is required when you don't have a static IP.

I guess you'll find find one of the above solutions to fit your needs.

Cheers
Bjoern