Ok, I'm going to try and remake my case for rejecting free will - apologies for length. One thing I want to make clear at the outset is that I don't think that the responsibility issue is relevant in this argument. Certainly, it will be affected by the outcome of the free will debate but the fact that responsibility carries moral importance for us does not mean free will must exist, unless we allow wishful thinking to dictate truth to us (I trust I don't have to explain why this would be silly). Anyhow, with that out of the way:
Lets use the following diagram to get a grip on the general situation -
World ---- (?)----> Will ----dn----> Bodily Action
It seems to me that both of us want to keep the Will in the debate (i.e. versus making us Will-less automatons) and so we have two possible directions of causation. They are World-Will causation, where the world (which may include one's mental contents) causally determines what one's Will is, and Will-Body causation where the Will affects a bodily change (which may include things like kicks, speech acts and mental calculations). 'dn' is used, like in my other post, to represent a decision chosen from a decision set D = {d1, d2,...} which represents possible decisions, constrained by logical and physical possibility.
The question I then want to ask is: why is dn chosen from amongst the other logically available alternatives?
(1) We could assert that we have truly free will and that World-Will causation plays no part in what decision is arrived at. In this case no world factor (beliefs, desires and values included) are able to constrain the Will, since we have rejected World-Will causation. But then we either have a completely random Will (which makes no sense given how orderly human behavior is) or a Will which somehow has constraints built into it. The latter case seems similarly unlikely to me given that our Will is extremely responsive to World factors such as one's environment, social upbringing and mental states. You thus need to allow for some degree of World-Will causation in order to explain how it manages to be so receptive to these factors.
In other words randomness is not enough to explain the correlation between World events and our Will. Even if the Will had information available regarding the state of the world and how best to achieve certain ends it's choice would still be random unless it had reason to make one decision over another. Information alone is not enough to eliminate the randomness of the Will - you need some kind of World-Will causation.
(2) 'Alright' you might say 'I concede that some World-Will causation takes place but I also maintain that the most important World factors are mental states, such as beliefs and values, which I take to be partly established by the Will, leaving room for freedom'. My response is that at some point the chain of '...-->World-->Will-->World' has to bottom out. If you bottom out with the Will you have randomness at the outset and then determinism later on as the causal chain determines what each subsequent Will decides. For similar reasons to above I still think that it is silly for our initial act of Will to be purely random - we are to World-responsive for that. It makes little sense to say that visual input of certain objects received by a baby has no effect on whether it forms a belief that they exist, leaving the belief up to random chance. This seems to leave initial World-Will causation as the better option.
(3) Finally, I think independent arguments can be made to suggest that important World factors which affect the will, such as beliefs (, desires, values, etc.), have their roots in World factors (some of which have been touched on by 3unitz and myself) like social environment, parenting, brain structure and genes. If these things are to affect the Will (which I think they must if we are to explain the orderly and World-relevant nature of our actions) then we have to concede that we are largely determined by the World, construed as above.
On a final note, Ockham's razor would suggest that we reject the metaphysically suspect, supernatural, causally isolated object that is Free Will in favor of a naturalistic, world-constrained will.