One problem with S_alanet's argument is that he seems to be saying that because we cannot prove whether or not we have free will, we must therefore have it.
By the same argument, I can claim that there is no free will, because you cannot prove either that there is or is not free will, and I think there isn't.
I am looking at this from a philosophical point of view. From the theological, or perhaps faith based point of view, He May Have A Point. S_alanet seems to be saying that free will is all in your mind. I am talking about free will from an ontological point of view. In this sense free exists, or it doesn't. Whether you know about it or not has no effect, in much the same way as we might say that Pluto existed before we proved that it existed, and Vulcan was proved not to exist even though we believed (for good reason) that it did.
Re: S_alanet, yes, it may be logical to act as though you have free will. But that has no baring on whether or not you actually have free will.
Your statement
"Predestination doesn't imply the lack of free will."
Should be
"Predestination doesn't imply you should act as though you don't have free will"
And this too I can argue with.
Believing in determinism doesn't imply that you should act differently than you should if you believe in free will. What you think and feel remains the same. What you feel is right and wrong remains the same. What will make you happy or unhappy remains the same.
If it happens to be true that we are predetermined to try to do what we think is right (or what makes us happy), it wouldn't imply that you can stop acting and everything will turn out okay. In fact, it would be impossible for you to try to do something wrong (or that doesn't make you happy). It would also be impossible for you to stop trying. (Please note that this is a conditional; there is an if there. I'm not saying that this necessarily is the case).
Also note that the lack of free will would not mean that we could not change things. It probably does mean that we cannot choose freely*. But we could very well change things, in much the same way as a meteor (completely devoid of free will) might well fall from the sky and kill you. We also have all kinds of fancy feedback loops that allow us to change what we think and how we think it; if there is such a thing as improvement (on any and every level), there is no reason to think that we cannot improve, even if it is the case that we cannot freely choose* to improve.
* Please note that I am not actually defining "freely choose", nor saying that the phrase makes any sense, or that it doesn't, or hinting at what it might mean if it does. I'm just saying that if you want to use the phrase "freely choose" or something of the sort, it ain't the same as "change".
S_alanet is using determinism as a worldview, while I have been using it as a objective theory that we are trying to prove/disprove.