http://www.technollama.co.uk/us-court-declares-gpl-is-a-contract :
[...]
In a strong declaration that online open source licences are contracts, the court declares:
“Defendant contends that Plaintiff’s reliance on the unsigned GNU GPL fails to plausibly demonstrate mutual assent, that is, the existence of a contract. Not so. The GNU GPL, which is attached to the complaint, provides that the Ghostscript user agrees to its terms if the user does not obtain a commercial license. Plaintiff alleges that Defendant used Ghostscript, did not obtain a commercial license, and represented publicly that its use of Ghostscript was licensed under the GNL GPU. These allegations sufficiently plead the existence of a contract.”