xtable formatting: RED for negative numbers?

I've been experimenting recently with the fantastic Sweave/xtable
combination for generating latex.  In the xtable vignette, I found this
great example of printing a ts object by months.

Is there a way to modify this code to generate RED numbers inside xtable for
negative results in x.ts?
Thanks in advance.

- Ken

# Sweave/xtable snippet below
--------------------------------------------------------------

\begin{table}[ht]
<<sample,echo=F,results=tex>>=
x.ts <- ts(rnorm(100), start = c(1954, 7), frequency = 12)
x.table <- xtable( x.ts, digits = 1 )
print( x.table, floating = FALSE )
@
\end{table}

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