As noted, only IObservable<T> and Observer<T> is in the framework, not Observable which carries all the extension methods of interest - e.g. to convert events. Next exercise is to create them oneself. On that task now...
Sadly if often unavoidable to implement something that will be included in the future. For example all those complex number implementation which are 'obsolete' in .NET 4. (Afaik the BCL in 4 has a complex-number implementation, right). Or in my project I've
some ugly 'cast'-hacks for generics wich are useless with co- and contra-variance. =(
Anyway, at least the foundation is there so you can make your code Rx-ready =)