For example the system they used at the college I lived in at the University of Newcastle, where you had to send in a paper form to register your mac-address to the ethernet outlet in your room, and weren’t able to use the internet on your laptop when visiting friends on the campus.

Be aware that using these commands to work around such a policy is probably illegal, though.
In snow leopard, spoofing your ethernet MAC-address is as easy as opening a terminal, and type sudo ifconfig en0 lladdr aa:bb:cc:dd:ee:ff

The address can of course be anything you want it to be.

For spoofing the address of your airport card you first have to disconnect from the network you are currently on, this can be done by clicking the airport-symbol in the menu bar, select join other network, type some random name, hit connect and then cancel. Now type

sudo ifconfig en1 lladdr aa:bb:cc:dd:ee:ff

And then you can reconnect, with your new address.

Update:

Captain Future has created an applescript for spoofing the mac address that looks very nice.  It can be found here. Thanks!

Related posts:

1. Installing Wireshark on Mac OS X 10.6 Snow Leopard
2. Useful BASH-command: !!