It is not immediately obvious how one can install weaver (thanks to balajis for telling me about it--see his comment to the Sweave speed issues).
Do the following as superuser for a system-wide install:
1. install digest (from CRAN)
2. install codetools from http://bioconductor.org/packages/1.9/omegahat/html/codetools.html
3. install weaver from http://www.bioconductor.org/packages/1.9/bioc/html/weaver.html
A clunky way to install on Mac OS X and Linux is: as superuser do
R CMD INSTALL packagetarball
I'll add information on how to use it later, but one can always consult the weaver manual/vignette.