<p>There are several people about (myself being one of them) who have been developing RIA for a while now using Flex and AJAX.  However Silverlight is now on the scene and worth picking up.</p>

<p>Question is, as someone who has never looked at .NET before, should I just dive in Silverlight and try and make up the bits I'm missing, or start somewhere else in .NET in order to get a foundation first?</p>

I firmly believe in learning foundations first. I would start off by writing console apps - get the hang of collections, IO, the language itself (whether that's VB or C#, or even something else).
It's probably worth learning a bit of "normal" WPF after that, on the grounds that it may well be easier to experiment that way than with Silverlight itself. (I can't say I've written any Silverlight myself yet.)
If you dive straight into Silverlight, when you run into problems you won't know whether it's because you're unfamiliar with the language, the core platform, or Silverlight itself. You'll take a while longer to get going with my suggestion above, but once you've got going it'll be a much smoother ride, in my view.