My co-worker Craig suggests that it will be a year or so before we have a real answer to that question. I tend to agree.
A hire/no-hire decision is based on many factors. Previous experience, references, and education weigh most heavily because it's easier to measure the value of those things. More importantly, they can be valued independently of what the prospective hire might say. Stack Overflow (SO) reputation can't yet be valued with measures external to Stack Overflow. Once it has been around awhile, that might be easier. For now, I think the vast majority of companies that employ programmers won't weight SO reputation much (if at all) in their hiring decisions.
There will be exceptions to this (of course). There are employers like mine, who are aware of Stack Overflow because we all read Coding Horror and Joel on Software (and in my case, managed to get on the Stack Overflow beta). If I'm in the interview rotation and a candidate mentions that Stack Overflow is a site they use regularly, it would give me a more positive view of them. Experience, references, education (and how well they interview) would still be the primary factors in my decision, but quality questions and answers in SO would only help.