|Don't ask to ask, just ask|
Well, IANAL, but I can't imagine that sort of clause standing up in court in the UK. Despite what makes a good story for the newspapers, UK judges tend to apply common sense, especially in common law, which covers most of contract. They would put such clauses on a par with a saw manufacturer claiming rights over chairs. There is some statute law, but that tends to protect consumers, for example http://www.oft.gov.uk/about-the-oft/legal-powers/legal/unfair-terms/. But other countries will differ.
Update: fixed minor typo
Update 31/5/12: An example of case law has just appeared: http://www.metro.co.uk/news/899768-boy-5-made-50-000-loss-on-spread-betting-website
Update 6/6/13: A critique of on line terms under EU law has been published by the British Broadcorping Castration: http://www.bbc.co.uk/news/technology-22772321
Update 2015-10-04 And if the Ts & Cs aren't clear, they aren't valid. http://www.telegraph.co.uk/finance/personalfinance/household-bills/11906547/Its-official-you-dont-have-to-read-the-Ts-and-Cs.html