This is actually a OS X 10.7 Lion question NOT a Snow Leopard question as it forced me to choose!
The description of how to disable IPv6 on the "" page is no longer accurate, as the option to choose "Off" from the menu item vanishes from the Configure IPv6 drop down menu. Found this when trying to help my sister debug a problem where she couldn't get an IPv4 DHCP address from her DHCP server, and replicated it on my own machine.
Didn't help (I'd tried it).
This solves it for MacBook Air - see above for details.
networksetup -setv6off Wi-Fi
It's important to get the capitalization correct, I suspect.
For Ethernet on Macbook Air using the USB Ethernet dongle, it's:
networksetup -setv6off USB Ethernet