Any owners of UK websites, what are you planning to do? The law seems pretty stupid and unenforcable, and something which would be better done by the browser. But it appears that individuals can claim compensation, so just because the ICO could probably never enforce it, doesn't stop some anorak taking you to court because the have a grudge.
Any opinions welcomed, I'm not sure what to do myself. If cookies are only used after a user logs in, then presumably a message on the registration page would be sufficient (the fact that they have an account proves they agreed to the message)? IANAL of course.