Sweave documents have corrupted double quotes

I'm attaching a file foo.Rnw and I'm hoping some of you might run it
through your R & latex systems to find out if the double-quotes in
typewriter font turn out as black boxes (as they do for me).  If you
don't use Sweave, but you have a system with a working version of R
and LaTeX, the file gives the instructions you need to use to process
the file. The

The file itself explains the problem. You can see the flawed output on
my web site

http://pj.freefaculty.org/latex/foo.pdf

I'm running Ubuntu Linux 8.10 with R 2.8.1 and TexLive 2007 (which is
provided with the distribution).

This is not a new problem, I noticed it two years ago while using
TeTeX on Fedora Linux, and so I doubt that this is specific to
TeXLive.  Back then, I took the path of resistance and stopped using
the typewriter font.  That is becoming inconvenient, however.

I would sincerely appreciate any pointers you have.

Paul E. Johnson
Professor, Political Science
1541 Lilac Lane, Room 504
University of Kansas