One of Microsofts main goals with Silverlight 2 for Windows Mobile was to get the compatibility between desktop and device. So you can essentially write a Silverlight application for the desktop and run it within the browser on the device (with the Silverlight device plugin of course). So you can use Expression to build a Silverlight UI for devices. There is no "mobile" version for doing this.
Throughout the mobile community many LOB (line of business) developers are worried that you lose the smart client "occasionally connected" architecture that you get with embedded Compact Framework based development when you use Silverlight for mobile devices. There is some worry with this although Silverlight for mobile once downloaded, does stay in memory and allows you to continue using the application. Of course during this downtime you can't communicate back to the server as per a smart client app. You can with Silverlight make use of isolated storage, as remember, Silverlight runs in a sandbox this is great news for Silverlight mobile devs.
Even so, this is still not a smart client application. So precaution needs to be taken and architecture reviews need to be extensive when choosing either Silverlight over an embedded alternative.
This also leads to the question: "when would I choose to use Silverlight over the Compact Framework?". Good question! And the answer is probably similar to that of why you would use ASP.NET over Winforms/WPF, although alittle bit more complex for devices.
Check out the following video from PDC where fellow MVP Maarten Struys, Microsoft Mobile Community Manager Constanze and Program Manager Amit Chopra with Microsoft Evangelist Giorgio Sardo talk about Silverlight 2 for Windows Mobile: