We have a .Net library that accesses some user hardware via a USB port. I read that Silverlight contains a subset of the .Net runtime. So my question is: is this subset enough to use the .Net library?
YES, YOU CAN
The update is only for people like me who found this question, here is the answer:
http://blogs.msdn.com/b/silverlight_sdk/archive/2011/09/27/pinvoke-in-silverlight5-and-net-framework.aspx
No, you cannot use the .NET library, which accesses the USB port from Silverlight.
The closest you can get is an application with SL4 Out-of-browser support with elevated privileges that COM components can use.