The 2.13.0 release has been quite solid, but some people expect an x.y.1 to roll out on larger installations for the next academic year. Of course, there have also been a sampling of minor bug fixes.
Looking at the NEWS file for today's beta build, there are indeed some minor bug fixes to graphics, edge cases in the class systems, and some accuracy improvements to the pbirthday function. (And hey, the birthday distribution is in R!) 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.
R-announce mailing list: R 2.13.1 scheduled for July 8
(Updated 12:00PM: Added links for pre-release Windows build and NEWS file, with thanks to Duncan Murdoch.)