For those of you holding out for WP7 apps to work directly on Windows 8, this
might come as a bit of a disappointment, but Microsoft has posted a guide on
MSDN for porting, manually, your apps from Windows Phone to Windows 8 Metro
The MSDN article reminds developers that Windows 8 apps need to support various orientations and aspect ratios, need to keep mouse and keyboard users in mind and make sure to rewrite the phone-call and other communication stuff.
Then there is a large list of equivalent namespaces and APIs that developers should use. For example, it suggests that developers use Windows.Networking.PushNotifications instead of Microsoft.Phone.Notification.
The article also reminds you that the model of coding for live tiles and notifications in Windows Phone and Windows 8 are similar, but with important differences.