Update: You can now download the Virtual PC and XP Mode bits from Microsoft:
http://www.microsoft.com/windows/virtual-pc/download.aspx

Microsoft announced today that Windows Virtual PC and Windows XP Mode have reached RTM and will be available for public download on October 22, 2009 - the same day that Windows 7 hits the streets.

Windows Virtual PC is the next iteration of Microsoft's client-side hypervisor solution (formerly Microsoft Virtual PC). Windows XP Mode is a pre-installed Virtual PC VM running Windows XP SP3 and can run applications seamlessly integrated into your Windows 7 desktop.

Check out the details here.