In spreadsheets it is usually easy to get red text for negative numbers using conditional formatting. This was the first thing I missed about spreadsheets when I started using LaTeX. For users of the xtable package in R, it is possible to conditionally format negative numbers to red via a little bit of regex work.
The function below takes the output of xtable as input. The output will be ready to paste into a LaTeX document, or to cat into a .Rnw file.