contracts do not have to be "physHical" to be valid. and no.... he has to use the money for what he said he would.
taking donations and speeding it on hookers and blow would not be legal. this would be called fraud.
I am indeed aware of this, techically, Theymos' only verbal contract was that donator's money would be used on forum upgrades. Donations were indeed not spent on anything except forum upgrades which you might consider overdue, but that should be irrelevant legally and morally. I know your hookers and blow example wasn't literal, but the funds were not spent on hookers, blow, personal expenses, etc.
I cannot give my opinion as to whether Theymos is morally obligated to refund donations, as morals are very variable from person to person. I can only respond to legally liable to refund donations, of which he is not. Will he refund donations, I have no idea, I'm assuming he will address it as well as your and other's other questions. I don't blame him for not discussing his possible ideas with anyone, as they seem to get leaked ASAP.
I am a Lawyer, and you can be sure that theymos DOES NOT have the OBLIGATION to prove on what he have spent DONATIONS he received without any kind of agreement (digital or paper).
And now is really the end of this history.
Regardless of if that is true or not, which pretty heavily relies on how the forum is registered (I don't want to disclose any details because I'm not personally sure) he has proved what the donations he recieved have been spent on. That comes in the form of donation money not leaving the forum's funds for the last two years, and the impending announcement of new forum software. Had he not told anyone about any of it until Monday when he said he would release the info to the public, we wouldn't be having this discussion.