Is an open source license considered an enforceable contract? Yes!
This is an interesting legal aspect… and a debate in various countries (I know that France is still ambiguous on that)… But at least in the US, a federal court has confirmed : an open source license is an enforceable contract.
This means that even though the two (or more) parties haven’t all signed said contractual document, it’s still considered as enforceable as a contract once a party decides to use a work that is licensed in such a way. At least in the USA.
Because of the legal uncertainty around things like the GPL in France, the French (yeah, we always behave different) have created an open source license that is designed directly as an enforceable contract. This license is called CeCILL and it is OSI approved. It is also listed as compatible with the GPL by the Free Software Foundation.
The world is getting better… one open source license at a time.