Even though there are only a couple of new R releases a year, the R Core Team is continually updating R in preparation for each new release. You can always download a prerelease version of R to see the most recent changes, but if you just want to keep informed of the changes that will be in the next release there's an easier way to keep track.
The R Developer Page provides real-time version of the NEWS and CHANGES files, both for the current version of R, and the forthcoming version. The CHANGES file lists changes for the Windows version and the NEWS file lists changes for all versions. There's also a link to list changes in both files. For example, to see what bugs will be fixed in the next Windows version of R 2.8 take a look at the CHANGES file for R-2.8-branch, or to see what new features are forthcoming in the next major version of R (presumably R 2.9) take a look at the NEWS file for R-devel.
There are also handy RSS feeds for each file. You can use the RSS feeds in an application like Google Reader to keep informed of upcoming changes without having to keep checking at the R Developer Page.
Find the files and the RSS feeds here: R Developer Page.
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