This is pretty cool. Check out this new feature in OpenSSH 4.3:
* Add support for tunneling arbitrary network packets over a
connection between an OpenSSH client and server via tun(4) virtual
network interfaces. This allows the use of OpenSSH (4.3+) to create
a true VPN between the client and server providing real network
connectivity at layer 2 or 3. This feature is experimental and is
currently supported on OpenBSD, Linux, NetBSD (IPv4 only) and
FreeBSD.