This blog is updated daily.
A general description is here.
abbreviate() gains a 'method' argument with a new option "both.sides" which can make shorter abbreviations.
A standard for specifying package license information in the DESCRIPTION License field was introduced, see 'Writing R Extensions'. In addition, files LICENSE or LICENCE in a package top-level source directory are now installed (so putting copies into the 'inst' subdirectory is no longer necessary).
str() is no longer slow for large POSIXct objects. Its output is also slightly more compact for such; implementation via new optional argument 'give.head'.