Even before tools like Reflector made it easy, decompiling .NET applications has become standard operating procedure. Common reasons to decompile an application include learning how compiler's translate code, working around a library's limitations, or outright stealing another person's work. In the world of web browsers, it is so common that "view source" is a primary means of learning how to program.
Setting aside the legal implications, decompiling a Silverlight 2 application is a simple matter. All of the files needed for a Silverlight application are stored in a single ZIP file called a XAP. It contains all of the XAML, DLL, and config files needed by the application. Once extracted, the DLLs can be opened with ILDASM or your favorite decompiler.
If you are interested in learning more about XAP files, check out Pete Brown's blog.