As announced by the R core team, R 2.13.1 has been released on schedule today. For those who build R themselves, updated source code is available now; pre-built binaries will propagate through the CRAN network over the next couple of days.
This update includes some minor bug fixes to graphics, fixes to some edge cases in the class systems, and some accuracy improvements to the pbirthday function. There are also a number of small improvements to the Sweave and vignette documentation systems, and other general improvements including some additional warnings that will help R programmers. One additional improvement of interest to R developers: the trace function now supports reference classes. The complete list of changes is in the NEWS file, linked below.
R Project: NEWS