I don't have the time to go through the code in latex.s
of the Hmisc
package, but until I do, feel free to wrap your chunks into centering environment. Not the cleanest solution, but it gets the job done.
\begin{centering}
<<results='asis'>>=
latex(tabular((Species + 1) ~ (n=1) + Format(digits=2)*(Sepal.Length + Sepal.Width)*(mean + sd), data=iris ))
@
\end{centering}
This produces a centered table.