Microsoft has said that XP Mode will be available through Windows Update on October 22nd, 2009, which is Windows 7's public release date. Then, presumably, the download and installation will all take place within Windows Update. (We will update this article then to reflect the new reality.)
Why is Microsoft not providing XP Mode in the base install for Windows 7 Professional, Ultimate, and Enterprise? I think it's because of where the application is aimed. It's not intended to be used for playing games or running applications that are CPU-intensive or time-critical. Instead, it's to allow small and medium sized businesses to continue to leverage their investments in software, including customized vertical applications, that might not have been able to run in Vista.
Thus, there's no reason for most businesses to stick to using Windows XP. A faster, prettier, more secure operating system awaits.