If your app is built on Silverlight today. Stop building things on Silverlight today.
Reasons are:
Time to market
Lower cost
Reuse MVVM
Reuse Silverlight controls
Reuse the Business and Data layer
The Biggest thing going for windows phone is:
.NET dev shops
Environment more productive than xcode
Note: WP7 is built on Silverlight 3 (not Silverlight 4)
Note: WP7 was built by a 'web team' not "Rich client app team', so some issues exist:
Bing map control relies on host page (authentication)
Avoid using System.Windows.Browser.dll in your Silverlight App (as it works only
OOB on desktop) and you will have to use a different control on Windows Phone 7
Others reasons are:
You can deploy your app without going through the appstore/market place. E.g. my SSW business app