People have found it to be anywhere between 400-600, but its response to light isn't directly equivalent to film. It's different with different amounts of light.
There are many threads devoted to this, do a search. In interlaced mode, yes, 1250 is about right, in progressive mode 640 is about right, but the response is *not* linear. You can't just set your light meter at 640 and expect it to return proper results, it all depends on the overall light level in the scene. The camera responds differently at different points in the exposure curve.
