proposed-updates is the mechanism used to prepare the point (minor) releases of stable (lenny) and oldstable (etch, which will continue to be supported until about a year from now). Packages are added to proposed-updates either automatically (in the case of security updates from security.debian.org) or manually by developers. Once rigorously tested, they are released into the main stable repository.
Of course (I hope!), you already have the security releases in your sources.list, but if you want an early start on the other proposed updates, and to help out with testing, you can also add in proposed-updates. Add this line to your /etc/apt/sources.list:
deb http://mirrors.kernel.org/debian/debian lenny-proposed-updates main contrib deb-src http://mirrors.kernel.org/debian/debian lenny-proposed-updates main contrib
Note: do bear in mind that these updates are still being checked. So while they should be fine to use, it's best not to do so on your Really Important Production Server. If you do experience any problems, report them using the reportbug package.