When you create a new list in the Assembly List Management dialog, the first time you open a brand new list Reflector offers to populate it from one of the versions of .NET on the machine.
This always used to offer Silverlight as one of the options. (Slightly annoyingly, each time Silverlight upgraded, you'd need to recreate the list because everything had moved, but at least it was quick to do this, thanks to Silverlight always being on the list.)
Silverlight no longer seems to appear. (I'm running 220.127.116.11)
It's possible to go and add all the Silverlight assemblies by hand, but since you need to do this every time a new update to Silverlight gets pushed out, this is slightly tedious. Any chance we could have the auto-population of Silverlight assemblies back please?