Trick question, you can’t, or not in any sane way that I’ve discovered. When an app using Microsoft Silverlight is running in the browser you at least have the browser’s print features, but a SLOOB (Silverlight Out of Browser, a new feature in Silverlight 3) does not even have that to fall back on.
This is unfortunate for business applications, though of course many applications are fine without print features.
Workarounds: well, you can press Alt+PrintScreen and paste into Paint…
Another idea is to generate a document in code, then let the user “download” it from your SLOOB to somewhere on the desktop, since finding a document in the app’s isolated storage would be a challenge. The user can then print the document at his or her leisure.
What this illustrates is that even in version 3.0, Silverlight is not altogether mature, especially for this new out-of-browser feature. I’m told that print support is planned in some future version – Silverlight 4.0 I guess. Software is always a waiting game.
Note this post is based on the Silverlight 3.0 beta; it’s just possible something could be added in the final version.