I tend not to upgrade R very often—running from 6 months to 1 year behind in version numbers—because I had to reinstall all packages: a real pain. A quick search shows that people have managed to come up with good solutions to this problem, as presented in this stackoverflow thread. I used the code in my mac:

# Run in the old installation (pre-upgrade)
# saving a list of installed packages
setwd('~/Downloads')
package.list <- installed.packages()[, "Package"]
save(package.list, file='package-list.Rdata')

# Run in new install
setwd('~/Downloads')
load('package-list.Rdata')
for (p in setdiff(package.list, 
          installed.packages()[,"Package"])) {
  install.packages(p)
}

# Final output
Warning messages:
1: In getDependencies(pkgs, dependencies, available, lib) :
package Acinonyx is not available (for R version 2.13.2)
2: In getDependencies(pkgs, dependencies, available, lib) :
package AnimalINLA is not available (for R version 2.13.2)
3: In getDependencies(pkgs, dependencies, available, lib) :
package asreml is not available (for R version 2.13.2)
4: In getDependencies(pkgs, dependencies, available, lib) :
package INLA is not available (for R version 2.13.2)
5: In getDependencies(pkgs, dependencies, available, lib) :
package graph is not available (for R version 2.13.2)
# Installing INLA
source("http://www.math.ntnu.no/inla/givemeINLA.R")
inla.upgrade(testing=TRUE)

From all installed packages, I only had issues with 5 of them, which require installation from their respective websites: Acinonyx, INLA (and AnimalINLA) and asreml. Package graph is now available from . INLA can be installed really easily from inside R (see below), while I did not bother downloading again asreml and just copied the folder from `~/Library/R/OldVersion/library/asreml` to `~/Library/R/CurrentVersion/library/asreml`.

Overall, it was a good upgrade experience, so thanks to the stackoverflow crowd for so many ideas on how to make R even nicer than it is.

P.S. 20100-10-14 Similar instructions, but including compiling R and installing bioconductor.