The Deployment Toolkit installation files can be downloaded from
Microsoft Downloads. Select the version appropriate for your host hardware. There is no charge, no licenses are required.
If you are upgrading from BDD 2007, read the documentation before installing. This should definitely be installed and tested in a test lab, using an environment comparable to your production, before installing in a production system.
A new version named MDT 2008, was released on March 20, 2008. If you are doing a new install, be sure you get this one. It's also shown as version 4.1. If you upgrade the previous version of MDT, it will require a reboot if you set up the PXE filter with WDS, and WDS is running. Otherwise it will not require a reboot. After upgrading MDT, you need to rerun the SCCM Integration (while the SCCM console is not running). For a more complete description of the upgrade, see Michael Niehaus' blog
article.