The Microsoft.Phone.Reactive assemblies are the ones that originally shipped on the ROM. The ones from the nuget package or Rx download are newer and apply a number of bug fixes
and performance tweaks. I would recommend using the 2.0 version from the download and exclude the on ROM version in your app at this point.
http://www.ThinqLinq.com http://www.LinqInAction.net - "LINQ In Action", The book is now available. Don't wait for the movie