Quoting Debian Squeeze release notes:
Some packages from StableProposedUpdates may also be made available via the squeeze-updates mechanism. This path will be used for updates which many users may wish to install on their systems before the next point release is made, such as updates to virus scanners and timezone data. All packages from squeeze-updates will be included in point releases.
Note that this replaces the functionality previously provided by the DebianVolatile archive.
In order to use packages from squeeze-updates, you can add an entry to your sources.list:
deb http://mirrors.kernel.org/debian squeeze-updates main contrib deb-src http://mirrors.kernel.org/debian squeeze-updates main contrib
The next time you run apt-get update, the system will become aware of the packages in the squeeze-updates section and will consider them when looking for packages to upgrade.
When a new package is made available via squeeze-updates, this will be announced on the debian-stable-announce mailing list.