What Is Original Sin?

Given my recent post on the matter, I've been thinking a lot about original sin and talking about it with different people. I've been a bit a frustrated by the vagueness of the doctrine. One friend told me that he never thought original sin implied (for example) the guiltiness of infants, but that it had something to do with the sinful nature or fallenness of mankind. I've heard different people suggest similar understandings of original sin.

Call such an understanding a "weak" view of original sin. A couple thoughts about the weak view:

1. No one I know of claims that Jesus was born with the stain of original sin. (I may not think that Mary was immaculately conceived, but I certainly think that Jesus was!) But Jesus was tempted, so he must have had a sinful nature in some sense...right? (Remember that what the NIV translates "sinful nature" literally means "flesh" - and Jesus clearly had flesh.) So it seems that, under the weak view of original sin, Jesus was born with the stain of original sin. But that is problematic.

2. Does anyone really dispute that we are born predisposed to sin or with sinful natures? And, if not, is the difference between proponents of a weak view of original sin and opponents of original sin just a semantic one?