Just read the last post in Scott Gu's great intro to Silverlight. Great tutorial. But it was the last one that really shocked me: porting a Silverlight app to Windows. Seriously, very little work involved. I think if you knew what controls to avoid ahead of time you could make it almost seamless. Shawn Burke has a discussion here about why the transition isn't seamless now.
For my money, I'd start with Silverlight and port to Winforms. Basing that on the amount of jobs for asp.net devs vs. jobs for winforms devs. Expecting that trend to continue into the Silverlight world. There's also a big "why" for porting apps from Silverlight to winforms (WPF)? For line of business apps, if your connection to your db is down then having a WPF version of your app won't help you much. No network, no workie. And if your connection to your db is down so often that you really need to have a WPF version of your app, then you have a bigger fish to fry.
Now, if we're talking about a mobile app, then disconnected might matter, but we have many other issues to deal with as well, so it's a big conversation.
Also, got my "Nice Answer" badge on StackOverflow. Achievements for developers, whoop whoop.