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A general description is here.
The canvas used when plotting on a jpeg() or png() device with a transparent background (most often by copying from a screen device with the default transparent background) is now white and not grey.
The "data.frame" and "factor" methods for [[ now support the 'exact' argument introduced in 2.6.0.
iconv() is again able to translate character strings with embedded nuls (such as those in UCS-2).
plot.lm() gains a new argument 'cex.caption' to allow the size of the captions to be controlled.
A series of changes make the CHARSXP cache introduced in 2.6.0 faster (and in some cases many times faster) in sessions with a large number (e.g. a million) of unique character strings.