When Rich Blake (firstname.lastname@example.org) photographed this sparrow in his Blythewood, SC backyard on 1/10/2011, his reaction was "what the H---". This matches the commentary of most experts so far. Could it be an oddball Dark-eyed Junco? A Junco x White-throated Sparrow? The throat and head pattern resembles the hybrid Junco x W-t Sparrows photographed by Norm Budnitz and William Byler, but note that the belly in this bird is gray, not white and the bill is dark, and the wings and back do not show any of the White-throated pattern.
The photos here are full-sized unenhanced crops, so no further detail is available. The bird was not seen the next day.
North Carolina Bird Photoscwcook@duke.edu