Quite how the law systems of the world view this latter kind of "contract", I'm not sure, but an awful lot of people rely on it - I have a feeling it rather depends on the details because people have suggested that the "EULAs" you "agree to" when installing software might not always be enforceable. But it hinges on the fact that nobody has the *right* to contribute to your wiki; they do so on your terms, or they don't do so at all.
IANL, but I have read something about copyright laws doing research for my own site. Contracts can be entered into by more means than signing a paper. Oral agreements can be a legal agreement (based on certain rule). Accepting an agreement by pressing an 'ok' button on a website can definitely be a legal agreement (otherwise banking on the internet would not work). I even had a website call me with an automated message that recorded my voice saying, "I accept." If you make it a clear and easily identifiable message that submitting information to your website is bound to some terms, than that is an agreement and can be upheld in a court of law (so long as the terms themselves are not illegal, ie: a contract to kill is not a legal agreement).