I'm currently working in the depths of Silverlight and we've recently come across a interesting issue when trying to deploy to a test environment.
As ever, everything was working fine locally, but when we deployed to a W2K3 box the Silverlight content of the site wasn't displaying. No errors, just nothing displayed. We checked the scripts and Xaml files were in place correctly and they were so we were a little puzzled.
Anyway we decided to use fiddler to check the traffic and that told us that when Silverlight was trying to load our Xaml file it was getting a 404 error. We double checked that the Xaml file was there, and indeed it was so immediately this smelt like an IIS configuration issue.
After some digging around the solution was to setup a MIME type in IIS for the Xaml being served. Big credit goes to Jim at Microsoft who has this blog post explaining the solution.