Unfortunately no, covariant return types aren’t supported in C# for method overriding. (Ditto contravariant parameter types.)
If you’re implementing an interface you can implement it explicitly with the “weak” version and also provide a public version with the stronger contract. For simple overriding of a parent class, you don’t have this luxury I’m afraid 🙁
(EDIT: Marc has a reasonable solution – although it’s pretty ugly, and method hiding is generally a bad thing for readability. No offence meant, Marc 😉
I believe this is actually a CLR restriction, not just a language one – but I could well be wrong.
(As a matter of history, Java (the language) had the same restriction until 1.5 – but it gained covariance at the same time as generics.)