More on How Assemblies are Downloaded in Silverlight 1.1
Assem in this thread on Silverlight.net pointed out that the downloader object isn't required to get assemblies down to the local machine in Silveright 1.1 (September alpha).
I didn't believe that, so I did a test. Guess what? He's absolutely right. Calling Assembly.Load(name) is sufficient to cause the assembly to be downloaded to your workstation. Thinking about it in retrospect, that makes perfect sense, as that is likely how the initial assemblies listed in the xaml file are brought down. However, I'd need to test myself to be sure of that :)
So, in the case of something like the assembly preloader I wrote about, the downloader object providers finer-grained control over the downloading of assmblies, but it isn't strictly required. If you want to load a single plugin, or a couple plugins, and they aren't overly large, you can just call Assembly.Load. If they are larger, which would probably necessitate the use of a preloader, you can use the Downloader object to get them for you, and then call Assembly.Load().