First of all, the right term here is "bundled", not "attached" (which implies to me that you think I was suggesting it might be a kernel module or something; I made no such implication).
Also, I was under the impression that there was more to silverlight than web apps... like Java, not everything is part of the browser, you can have full blown applications as well. Isn't that also true with silverlight? If so, then I doubt a full silverlight implementation would _just_ be a browser plugin.