Let's start with these points... 1. Opening RDP to the world, to administer from one remote device is a bad idea.
Let's start with these points... 2. Although RDP is encrypted, there is no verification of the server's identity by default - this makes it possible for man-in-the-middle attacks.
Let's start with these points... 3. AFAIK rdesktop doesn't support TLS, so enabling it to reduce the risk of a man-in-the-middle attack is not an option.
Let's start with these points... 4. VPN is good. Off the top of my head, OpenVPN and vpnc (Cisco compatible VPN client) are both available for Maemo, so there is no reason not to use it.