Actually ever change my credit card company has made has been notified by post, and contains an option to retain the current T&C at a cost of the card 'locking', the old T&C applying during the time you pay the balance off, and no further changes being made.
I'm assuming that if these sharks don't think the idea of posting it on a website and basically saying 'tough' to anyone who didn't notice would fly, then it probably won't. *they* dare not run the risk of such contracts being rulled out by a court, so they make sure such a ruling would not effect them.
ISPs are a different kettle of aquatic swimming things, but banks seem to know which way it would go, I reckon the likes of ISPs assume no one would risk the costs going to court, and small claims courts don't set a precedence (I think), cheaper to have iffy T&C and settle when challenged, the majority just pay up.
its the same as civil parking 'fines', essentially unenforceable, but many people will pay up.
Also I hear a company that tries the "you can't sue us, because we said so" over here will fine the court disagrees and any penalties will be bigger.