Since the release of Debian 3.1, support for AMD64 has -unofficially- been included. CD and DVD images will be made available on cdimage.debian.org. If you visit packages.debian.org and search for a package you will notice (since a few weeks ago) that it also lists the amd64 versions, including links to download the packages from one of the mirrors. This will continue to work for sarge, but may break for short times while amd64 gets integrated into the main Debian archive. More information can be found on the Debian AMD64 homepage.