In theory its quite easy to share an internet connection in OSX. In reality it doesn't work like Apple say.
Go into Preferences>Sharing, and click on Internet Sharing. You can share from any network connection to your choice.
The default is to share _from_ ethernet, but click in the drop box and you see all the options
Select where to want to share to, in this care Airport, and then click the Airport Options box
You need to call your ad-hoc wireless network something, and then unless you have a really good reason, enable encryption. Don't select 40-bit you may as well leave uencrypted for all the security you get. Give it a password, and click OK.
Back in the Internet Sharing panel click the box to the left of the words 'Internet Sharing' and you get the standard 'are you sure box'. Click start.
If its worked OK the usual Airport icon change from
However! This doesn't work for me. For some reason I have to create a wireless network first.
To do this click on the Airport fan and select 'Create Network'
In the dialog box, put in some details
and the click OK. The Airport fan changes to
then go back to the sharing panel and click to start Internet Sharing.