I'm using latest Ubuntu Linux on my laptop, and it just did this out of the box with some setting in the Network Manager GUI.

I had the laptop connected to the net via wired ethernet, and enabled the wireless connection in connection sharing mode. Unfortunately I don't remember what option that was in the GUI. I do remember when selected it automatically provided IP forwarding, NAT and local DHCP/DNS.

After that I saw the laptop wi-fi on the N900's list of connections, picked it, and the N900 had net access via the laptop. Nothing more to it. I was impressed that it was built in to Ubuntu and didn't need me to fiddle around, and that it just worked.