Folks, the reason you are unable to retrieve the RSVPs is a known limitation/feature of Entity Framework 1.0. By design, EF 1.0 does not do implicit lazy loading by default. You have to call the Load() method.
Here's how you get it to work:
public bool IsUserRegistered(string userName)
bool isRegistered = RSVPs.Any(r => r.AttendeeName.Equals(userName, StringComparison.OrdinalIgnoreCase));
Like you, I am not currently able to upgrade to Visual Studio 10 & EF 4.0, but this should do the trick for you. I had to bust out my Entity Framework book to find this.
For more info:
see O'Reilly's "Programming Entity Framework" by Julia Lowry, 1st ed, page 469.
Or Google "Entity framework lazy loading versus deferred loading"