The Google AdSense publisher contract is a form of Adhesion Contract [en.wikipedia.org]. Take it or leave it. Note that I didn't say it was ethical or "not evil" -- just that these kinds of contracts have existed for a long time, and what Google's doing probably isn't unique.
And indeed, a contract can be binding without signed paperwork. All that's necessary is acceptance of benefits conditionally offered.
Though if trends continue, a lot of small AdSense publishers will be able to claim the contract is void for lack of consideration! (Or is the mere *possibility* of being paid someday sufficient consideration?)