UPDATE: Latest version is here: http://silverlightuk.blogspot.com/2008/10/silverlight-2-rco-version-of-spectrum.html
So I've just finished version 1.0 of my Silverlight spectrum emulator.
At the moment there is no keyboard interaction, or ability to load games (it is currently only running KLAX), but it proves Silverlight can emulate a spectrum
I will figure out this side of things so that a fully fledged emulator will work in time.
I will put up the code at some point to CodePlex. I used the ZX360 project on codeplex as my base code. Thanks to them for posting their code, and thanks to Joe Stegman for his blog article on Dynamic Image Generation (which allowed me to figure the final bits).
I also have a WPF version which I will post at some point. Emulator can be viewed at http://silverlight.services.live.com/invoke/65017/Spectrum%20Emulator%20Version%201/iframe.html
More versions and details to follow but enjoy for just now.