So was testing virtual machines under VMware ESX 3 today, and with no special configuration on the ESX server itself, I was able to set up tunnels on a variety of Guest OS machines using their standard example configurations. With Ubuntu Server 8.04 however I did have to modprobe for "ipv6" and "sit" after it booted up from a fresh install.
ESX 4 and greater now have support for IPv6 built into them. IPv6 is disabled by default, but enabling it just takes a mouse click and a reboot of the system. After doing that, you can login to the system console over IPv6 (and it seems IPv6 is preferred if the address exists)