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A general description is here.
R CMD INSTALL/check and tools::writePACKAGES accept a wider range of compressed tar archives. Precisely how wide depends on the capabilities of the host system's 'tar' command: they almost always include .tar.bz2 archives, and with modern versions of 'tar' other forms of compression such as lzma and xz, and arbitrary extensions.
gzfile() looks at the file header and so can now also open bzip2-ed files. It is planned in future to extend this to other forms of compression such as lzma and xz.
There is the new option of save(compress = "bzip2") to use bzip2 compression (which will be slower, but can give substantially smaller files). load() can read such saves (but only as from this version of R).
detach() now has an argument 'character.only' with the same meaning as in library() or require().
R CMD INSTALL has a new option --data-compress to control the compression used when lazy-loading data. THe default is 'gzip': 'bzip2' will give ca 15% better compression at the expense of slower installation times. More methods are planned for future releases.
methods(f) used to warn unnecessarily for an S4 generic 'f' which had been created based on an existing S3 generic.
Names of datasets could clash with temporary filenames used when running examples, causing errors.