In case you weren't watching, Wikileaks has obtained and released a large number of secret diplomatic messages from the US. Wikileaks is hosted in Sweden, and run by an Australian. Therefore, US law does not apply directly to them. But it does apply to the source(s) who leaked the documents to Wikileaks.
As a pre-condition for having one's US security clearance activated, one must sign several documents acknowledging one's having read and understood the legal penalties to which one may be subjected if one discloses classified information either (a) accidentally, or (b) knowingly and willingly. These penalties can be severe.
So, to the person(s) who stole the documents, one message. If they catch you and punish you, you can't say you didn't know it was coming.
For the rest of you, another message. It will probably surprise you to know that it is illegal under US law to classify a document without proper authority, as described in written classification guides. In particular, it is illegal to classify a document simply because its disclosure would cause embarrassment. This was enacted to correct abuses of the system of classification in the previous century.
So why does the US mark some information as classified, anyway? Information that is the property of the people is kept from the public domain (classified) in order to protect the people from harm. In other words, if you're an American, you are a co-owner of the US government, and all its information, classified or otherwise. It's just that, for example, you really don't want to know how to operate a nuclear weapon, because even though you would never use such knowledge, someone else, who wants to incinerate your town, just might.