Given that Silverlight is a subset of WPF, there are many potential problems.
First, I would ask why you want to do this, since they are usually used for different purposes. If you decide to make a port, then you need to do a namespace analysis to verify that all the namespaces that you use in your WPF application exist in the Silverlight runtime. If you use things that are not supported in SL, then you will spend a lot of time re-recording these parts.
Other issues that I know about are that Silverlight 3 runs in a sandbox, so you cannot use a drive, hardware on a box, etc. You are also limited in relation to any requests made over the network, as they must return to the hosting domain or use the cross-domain policy file.
Silverlight 4 , WPF, .
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