After several months as of testing, Backports is now an official Debian service available at backports.debian.org.
“Backports are packages from the testing distribution recompiled for the current stable (or even oldstable) release to provide users of the stable distribution with new versions of certain packages,” the Debian announcement explained, “like the Linux kernel, the Iceweasel browser or the OpenOffice.org suite, without sacrificing the overall stability of the system.”
The archive has 528 packages backported for the Lenny distribution.