I think this is somewhere in the mess that was SCO vs IBM
EDIT: this also seems relevant
"See Carson v. Dynegy, Inc., 344 F.3d 446, 452 (5th Cir. 2003) (“Assuming, arguendo, that a nonexclusive license was created, whether such a license was irrevocable rests solely with whether [the plaintiff] received consideration.”). See also I.A.E., Inc. v. Shaver, 74 F.3d 768, 775–76 (7th Cir. 1996) (discussing the existence of explicit and implied non-exclusive non-contractual licenses)."
“A non-exclusive copyright license (such as most FOSS licenses) can be revoked at any time only if there was no consideration involved. The United States Federal Circuit Court of Appeal took this on in
Jacobsen v. Katzer in 2008 and ruled that there is consideration exchanged in the use of FOSS by a licensee. This indicates that an FOSS license that's silent on revocation is likely revocable only for violation of it's conditions”