I include my SO profile directly on my resume and consider it to be just as important as the education. It gives the employer a way to look into how you communicate and what your current level of knowledge is.
If you're interviewing for a really good company, likely you will be interviewed by your future peers. Depending on the type of job, if your future peers don't know what stackoverflow is there is probably a problem with that company and you probably don't want to work there.
Also I wouldn't say that I had X rep, instead I would say that I am within the top 0.01% of SO users, or whatever that number is, and that I am ranked at the top of technology X, Y, and Z which is what you are looking for.