CSV2XLS is a command line utility that enables the creation of xls files across platforms. It supports multiple sheets, formatting, and the choice of fonts. It requires python. There is also an R/S+ plugin available, which is called DATAFRAMES2XLS.
WARNING: On Ubuntu 13.04, the .deb file almost broke my system! I got a 'package is of bad quality' warning, which I ignored (bad, bad move). I found that Software Center was doing a lot more stuff than usual, and then my Compiz just crashed, which is when I aborted. I later found that a lot of packages were installed, notably everything depending upon Python. Seems like a useful tool, and I hope the dev fixes it.