IIRC I got a text along the lines of "You're now connected to (whatever network) as part of 3 Feel at Home this will come out of your usual allowances" then another telling me how much it would cost for out of bundle stuff.
Be careful if you happen to be near a border. I connected to a Belguim network a few miles inside France that ended up costing a few quid as I was using Google Navigation at the time and as I was driving ignored the texts telling me that I had and how much it would cost!