Does PostSharp work on Windows 8 (or more correctly, with .NET 4.5)? The short answer is: not yet.
I tried to get it to work, I really did. I tried using the PostSharp Silverlight assembly. I tried installing with NuGet and I tried directly from the PostSharp installer. I tried it with a fox, I tried it on a box.
I'm guessing that SharpCrafters is going to wait until Windows 8 / .NET 4.5 is more stable and closer to release before worrying about updating PostSharp. SharpCrafters is a very small company, so they probably can't afford to spend resources on updating their product to work for a framework and operating system that aren't even used by the general public yet, which is totally understandable.
Chad England has confirmed in a blog post that you can get PostSharp working in VS11 with non-.NET 4.5 projects.