Currently are a lot of discussions and confusion related to Windows 8 and support for several technologies
First of all, it is important to know that Windows 8 will not support .NET 4.0. Some companies which already invested in .NET 4.0 seem to be scared about this. Fortunately this fact has nothing to do with some technology deprecation (MS never did that without giving support for 10 years (at least as I remember).
More interesting is that .NET Framework 4.5 is an in-place update that replaces .NET Framework 4 (rather than a side-by-side installation). This has nothing to do with Windows 8. NET 4.5 is (designed to be – look here for breaking changes) fully backward compatible with applications built for .NET 4 (.NET 3.5 and .NET 4.5 will be side-by-side). .NET 4.5 is build with version 4.0.30319. This is technically the same version as .NET 4. .NET Devs know that this practice is used for so called in-place updates.
So, Windows 8 will be delivered with .NET framework 4.5 and this will by default provide support for .NET 4.0 applications, because 4.5 is technically 4.0 main stream.
Here is overview of .NET delivery at various windows operative systems.