I think it's reasonable for an employer to ask for it, but I don't think its reasonable for that to be a make-or-break qualifier.
Joel mentions that a high rep equates to getting high paying jobsJoel mentions that a high rep equates to getting high paying jobs but by his own logic, unless you're under employedunder employed you may not have time to flex your mental skills and get a large amount of rep. So the man who waxes poetic about Stack Exchange (and rightfully so) admits that it's really an indicator of both high employability and under employment.