like (I think) many others, the extension I am writing for visual studio 2010 beta did not even load on the new visual studio 2010 RC. After searching and asking around, it looks like the main breaking change for me was that I was importing an IServiceProvider type, which in the RC needs to be imported as SvsServiceProvider.
all I needed to do was change form this:
[Import(typeof(IServiceProvider)]
public IServiceProvider provider;
To this:
[Import(typeof(SvsServiceProvider)]
public IServiceProvider provider;
you can see some other changes here.