Even stable is updated once in a while. Those updates are called "Point Releases". They usually incorporate the security fixes released until the time of the update and fixes for grave bugs in the current release. They are prepared by the Stable Release Managers (SRM). As of February 2009 they should happen at about every two months. oldstable is subjected to a similar handling until it is considered as end-of-life, although the periods may be longer.
Debian Etch 4.0r4 (name Etch and a half) was the first point release to introduce improved hardware support, through optional packages.
Current point releases
Previously, the initial release had r0 appended, but normally omitted: for example Debian Etch started off as 4.0r0. Point Releases increment the digit after the "r" to specify a certain revision of a release with updates included.
Announcement of Point Releases
Point Releases are announced on:
the News section of the Debian website (look for distribution updates),
Details are also available in each repository, like http://ftp.debian.org/debian/dists/stable/ChangeLog (or http://ftp.debian.org/debian/dists/oldstable/ChangeLog for the oldstable distribution).
How to propose an update for a Point Release
Please refer to the Debian Developer's Reference section on uploads to the stable and oldstable distributions.