The sources for R-1.9.0 are now available via rsync at rsync.r-project.org. You can, for example, use rsync -avC rsync.r-project.org::r-release ./R-1.9.0 to obtain a copy of the sources. Binary distributions for platforms such as Windows and Mac OS X will be available from the CRAN sites in a few days. I see that Dirk Eddelbuettel has already uploaded the binary Debian packages to the main Debian archives.