Yikes! My son does really well academically- but his handwriting is pretty awful!!!
Academically gifted children think they can get away with murder regarding handwriting (and they ususally do!). Do not buy that; my son was exactly like this until last January (2006) when we started him with an English tutor and in 2 months from awful handwriting he acquired the most fantastic joint handwriting anyone could hope for (not to mention his marks in English practically shooting though the roof at the same time!) Hence handwirting can be fixed. The only downside in our case is that for the first 40 days or so his hands were constantly ink-stained as she made him use an old fashioned pen!!!
Personally, my handwriting is absolutely horrible (and not always eligible) and I suspect that the reason behind it is that they did let me get away with it, and I knew that they would! We thought that in the case of my son it was heriditary, but there you go.
Having said this, as for the SATs I very much doubt that any school would shoot itself in the foot by subtracting marks for bad handwriting (so long as it is eligible) or even spelling in the case of Maths and Science.