Reactive Extensions Release Candidate is out and it now supports .NET 4.0 (except few features) along with 4.5. It also has several improvements in the way it deals with time. The Rx Team has announced the changes from the beta on their blog.
Regarding the .NET 4.0 support, the team had this to say -
Quite a few customers asked us about support for .NET 4.0, so we brought it back in the RC, allowing users to move to .NET 4.5 at their own pace. Keep in mind though, some parts of Rx rely on advanced .NET 4.5 capabilities (such as ExceptionDispatchInfo) and provide a better together story with other .NET 4.5 features (such asasync/await), so we still recommend to try out .NET 4.5 whenever possible.
Rx deals with time at various levels through schedulers and some LINQ operators like Timer(). However handling time can be complex and earlier implementations of Rx did not deal with some of issues like time drift leading to problems with handling absolute time. The Rx 2.0 RC uses a new implementation that promises to be better. You can watch this Channel 9 video of Bart De Smet explaining this in more detail.
The pre-release package is available both as an MSI installer and as a set of NuGet Packages.
Reactive Extensions (Rx) is a library for composing asynchronous and event-based programs using observable sequences and LINQ-style query operators. If you are new to the library, you can also read the MSDN article “When Will You Use Rx” and refer to the various resources on MSDN.