I'm not sure that the concept of free will is incompatible with determinism.
The important thing is that our decisions be made by us, with minimal
interference from unrecognized manipulation by other conscious beings.
Another important quality to free will is the ability to shift our goals based
on our own analysis of circumstances. Free will means being able to
independently develop goals without having those goals assigned by an outside
agency. I think the reason people are so concerned that our decisions be
"unpredictable" is to make it more difficult for us to be placed in situations
designed by another where we have no control, or even knowledge of loss of control. Indeed, it may be that the decision making process is too chaotic in nature to make perfect prediction of action an effective strategy, yet it could still be completely deterministic.
As a related aside, I was working on a Traumatic Brain Injury unit in January, and one of the things we had to do was assess the level of awareness/consciousness our patients possessed. This was difficult since many lacked the ability to speak at first, even though awake. What we would do is test for the presence of low probabilistic, contingent activity. We often would observe first, to see what novel low probability actions the patient would repeat. These were not very predictable (as one might expect from a low probability occurence). But really what we were looking for were signs of novel, low probalistic behavior that were signs of new internally generated goals. The novel portion was important, because stereotyped behaviors can be well preserved, without any sign of a guiding consciousness. For example, some people in a persistant vegetative state will sign their signature if you place a pen in their hand...but nothing else. I think I've strayed a bit off topic here, but thought it might be interesting.