The courts have said that it’s not acceptable.

The Ninth Circuit disagreed heavily with the original ruling, saying that it was not reasonable to expect Douglas to check the company’s web site every day just to see if the terms of service had changed. “Parties to a contract have no obligation to check the terms on a periodic basis to learn whether they have been changed by the other side,” wrote the judges. “Indeed, a party can’t unilaterally change the terms of a contract; it must obtain the other party’s consent before doing so… This is because a revised contract is merely an offer and does not bind the parties until accepted.”

But what if the original user agreement involved signing away rights to be notified of subsequent changes? There is some question as to whether this ruling would also affect that type of agreement, but as Eric Goldman of the Technology & Marketing Law Blog says, it’s relatively safe to assume that the decision applies to this situation, “despite contract provisions putatively permitting unilaterally posted website amendments which put the onus on users to check back frequently for updates.”