Doing some new R packages, seems like the following changes should be make to the spec examples in http://fedoraproject.org/wiki/Packaging/R:
Source0: http://cran.r-project.org/src/contrib/%%7Bpackname%7D_%%7Bversion%7D-%%7Bpac...
URL: http://cran.r-project.org/web/packages/%%7Bpackname%7D/index.html
BuildRequires: R-devel, tex(latex) (though only for F-9 and later)
Also, the noarch spec requires R but the arch doesn't. This may be correct (arch probably depends on R libraries), but you might note it in the summary of differences section.