on my n800 nothing at all. i just searched for the wifi connections selected the one, voila. on the Imac nothing at all either. just started browsing. i could ask for the settings of the imac and airport to be sent to me.
but the obstacle you describe might indicate either an old style adsl connection (the one requiring a log-in authentication ot the subscriber) or cable-tv internet : in finland, its impossible to connect a secundary device to internet on some cable modems...policy of the cable provider (again specific of the early wifi age, when internet providers feared to see clusters of users for free sucking on one point). last possibility: no internet connection: wifi works fine,but you re stopped. there's a specific notice from the connection wizard on the tablet though. happened to me once.
hope this helps...