Hmm, again, all theoretical, but, I think you could, with some skill make an agreement that would work to transfer copyright via an electronic writing.
You also cannot transfer ownership of property that does not (yet) exist (which is essentially what copyright is).
In most cases, the issue would likely resolve on the situation being viewed as a work for hire. But, I suspect that it may in fact be possible to transfer future rights. My brief look at some of the cases and issues suggests that you could have an "expectancy" interest in the future work, which essentially means that if the work is never created, then the site has no right to recover. Once created though, the assignment would "spring into existence," and be valid under this theory. The cases I looked at mainly dealt with right of renewal in copyrights and their assignments, but it seems like a plausible theory. I did see one case, though, that expressly rejected this idea. It would take way too much time to do a full search and to research the case law to see what the majority/minority take on this issue is, but it looks at least plausible as a theory.
However, I don't think you would even need to go there. Don't look at the TOS as a one time event purporting that once the member agrees, all future copyright-able submissions are transferred to site. I would think a way to address this would be to say something like, "Member agrees that by hitting "Submit Thread" button, the work contained in that post and any copyright is transferred and assigned to Site in exchange for Site's publication of the work." Essentially, it would make the submission of the already created "work" (created in the editor) a term of acceptance. And there would be no doubt that the work is in fact in existence before the acceptance. It would make each click of the submit button evidence of the agreement (though, I would think that if you really wanted to make this as enforceable as possible, you would take more care in drafting and probably make the user take an affirmative step like checking a box to signify agreement with the TOS overall, at a minimum). Seems, from a theoretical point of view, valid terms of contract and under 17 U.S.C. § 204 and 15 USC § 7001 a sufficient writing to create a transfer.
In the U.S. you would need to specify the actual works involved, which again, a click-through ToS probably would not satisfy.
The above solution takes care of this, too- the scope is what the member submits. Seems that this covers any problems with "proof" of scope.
Non-exclusive grants on the other hand need not be in writing, it can simply be implied
This is quite true and probably why this solution is the one mostly taken. I am just thinking through what is possible and would be at least arguable a valid and effective way to accomplish a transfer IF someone wanted to.
You'd also want to file with the U.S. Copyright Office (which would require a notarized form) to prove the transfer took place, should you later get sued.
This goes into recordation of the transfer. This would not be required, but even if someone wanted to do this (which I would only imagine would arise if the "work" had some particular value), it would not be terribly onerous to do.
A separate issue that might bear on this whole question (and one that I have not looked at all really) would be the impact of the site or the forums on the site being construed as a "collective work." Take a look at this:
"§ 201. Ownership of copyright
(a) Initial Ownership. — Copyright in a work protected under this title vests initially in the author or authors of the work. The authors of a joint work are coowners of copyright in the work.
(b) Works Made for Hire. — In the case of a work made for hire, the employer or other person for whom the work was prepared is considered the author for purposes of this title, and, unless the parties have expressly agreed otherwise in a written instrument signed by them, owns all of the rights comprised in the copyright.
(c) Contributions to Collective Works. — Copyright in each separate contribution to a collective work is distinct from copyright in the collective work as a whole, and vests initially in the author of the contribution. In the absence of an express transfer of the copyright or of any rights under it, the owner of copyright in the collective work is presumed to have acquired only the privilege of reproducing and distributing the contribution as part of that particular collective work, any revision of that collective work, and any later collective work in the same series."
Anyway, like I said, it is not necessarily the case that this is the "best solution" or one that would be recommended. The non-exclusive license approach seems easier and likely appropriate for most folks. But, if you wanted a site that did transfer copyright on submission, my instinct is that I think it can be done.