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A general description is here.
The parser no longer marks strings containing octal or hex escapes as being in UTF-8 when entered in a UTF-8 locale.
On platforms with cairo but not Pango (notably Mac OS X) the initial default X11() type is set to "Xlib": this avoids several problems with font selection when done by cairo rather than Pango (at least on Mac OS X).
Parsing strings containing both octal/hex and Unicode escapes now gives a warning and will become an error in R 2.12.0.
The X11() device with cairo but not Pango (notably Mac OS X) now checks validity of text strings in UTF-8 locales (since Pango does but cairo it seems does not).