With separate downloads for 64-bit and 32-bit versions of Windows 7, the download also requires an authentic copy of Windows 7 or Windows 7 upgrade, and is only supported on more recent Intel-based Macs; no Macs produced before 2007 will support Windows 7, as outlined in this Apple support page.
If you're upgrading your Mac's other side from Windows Vista to Windows 7, you'll first want to download and install the Boot Camp Utility for Windows 7, which "safely unmounts the read-only Macintosh volume on Microsoft Vista."
Let us know in the comments if you have any issues with the update.
Note: If you are already running Windows 7 on your Mac in Boot Camp, you can simply run Apple Software Update while booted into Windows to get the updated Win7-friendly drivers.
[Hat tip to MacRumors and the Apple Support RSS Twitter feed]