A number of questions were raised concerning the grant of license  
rights under open source licenses, and whether or not they can be  
revoked or amended at will.  This is deeply complicated under  
Copyright law, because the licenses in question give express rights  
to make derivative works, and with that comes additional issues.   
Even the purest cases are complicated, far more complicated are the  
actual licenses in play -- even simple MIT.

First: all a license is, is a covenant not to sue for infringement of  
exclusive rights.  A contract including a license may have other  
provisions, and the longevity of the license may depend upon other  
provisions, but at the end of the day, the LICENSE is an abstract  
thing, different from any writing or contract or other abstract thing  
related to licenses: a conditional or unconditional covenant not to  
sue.  All the rest is just plain ole' contract - applying state law  
in the US.

The license, however, under US law has two federal components: (i) if  
the license is an exclusive license, it is not enforceable unless  
signed by the parties; (ii) the grant of rights is a grant of some  
slice of the rights set in Section 106 of the Copyright Act,  
including essentially the right to reproduce, to distribute and make  
derivative works.

Open source licenses are rarely exclusive, but the grant of a right  
to make a derivative work has an interesting consequence.  A *NEW*  
copyright is created in the derivative work (which is the whole thing  
including the changes), and unless a contract provides otherwise, the  
*NEW* copyright will be owned by the deriver (or the commissioning  
agent if a work made for hire), not the owner!

Now, lets take some licenses in pure contexts:

     1) Phone call: "Hey jim, great game last night.  By the way, you  
can reproduce and distribute CODER as you please -- you told me you  
just wanted to mess around with it.   No problem, just like I tell  
anybody else, go ahead."

Not an exclusive license, so it doesn't have to be in writing.   Not  
a right to make derivative works, so not complicated.  An oral  
license is enforceable, although there are risks associated with the  
use, since ultimately a jury will decide whether the conversation  
took place.  But if the jury believes the conversation, the legal  
effect is the grant of a license.

     2) Phone call: "Hey jim, I hate what you did yesterday.   
Everything is off on your reproducing and distributing CODER.  Stop  
it or I will sue.  I will follow up in a writing and demand letter  
from my lawyer."

Licenses under law are revocable at will, absent certain exceptions.   
Although state law governs that question, and it may vary state to  
state.  The fact that there was no reasonable appreciation by the  
grantor that Jim would rely on perpetual rights, and change his  
position as a result (he just wanted to "mess around") is a strong  
indication that the license is probably revocable.

Probably not necessary to revoke in writing, but you face proof  
problems unless the jury believes the revocation occurred.  Some  
states require revocations of offers to be in writing, and others  
require it in writing when the license was in writing.

Assume it were otherwise.  That Jim reasonably relied upon the  
assumption the license would be perpetual, hired a few people and  
bought machines to use CODER.  Further assume that the grantor could  
be expected to know that Jim's conduct were reasonable.  In most  
states, these facts could be grounds for equitable estoppel that  
would preclude revocation of the license.  Other grounds to make an  
otherwise revocable license irrevocable include the transfer of  
valuable consideration or an interest in the work for the license.   
Again, state law governs and I wouldn't presume that the result in  
one state would be the same as another, let alone from nationality to  
nationality.  Signed writings can make a license irrevocable in some  
cases.

Note that revocation of a license is sometimes legally different from  
its termination.  A license once revoked, is the legal effect of  
dissolving the license.  ANY USE THEREAFTER is infringment, period --  
and because of the knowledge of license and revocation -- willful to  
boot.  A license may be terminated under various circumstances set  
forth in an agreement, which may require conditions to be satisfied.   
It is possible in some termination cases that a declaration by a  
court as to its termination is required before an infringement occurs.

Finally, assume that the license was broader, permitting not only  
reproduction and distribution, but also derivation.  Assume I create  
a derivatie work: FORKEDCODER.  Your termination or revocation was  
effective AFTER FORKEDCODER was written.  What is the effect of the  
termination of the license to CODER?  Quite possibly none at all.  
possibly the death of the fork.  this is a generally cutting edge  
area of the law, and we won't know until courts have decided.

The long and short of this, is that these questions are AMAZINGLY  
fact and jurisdiction dependent.  It would be ridiculous for a non- 
lawyer to presume, without specific advice of counsel who has  
analyzed the particular facts, they knew the answer one way or the  
other.

End result:  It is risky in the extreme, to presume without sound  
advice that a license is irrevocable -- either as grantor or grantee.