Jeff Prosise has published this amazing post on page orientation in Windows Phone 7.
When you write browser-based (or out-of-browser) Silverlight apps, you don't have to worry about a user turning his or her monitor on its side. The phone, of course, is different. Silverlight for Windows Phone introduces the concept of page orientation, which tells you whether the phone is upright, upside-down, or on its side. Most apps don't have to do much to rotate the entire display when the phone itself rotates; Silverlight does that for them. But some apps will need to know when the phone has rotated so they can customize the UI programmatically.