I am afraid things are going to get confusing in the next "release" of what is being referred to as .NET 3.0. I will do my best to demystify what I think is aleady becoming a complicated topic.
.NET Version
Historically, the version of the framework has been in-line with the CLR version. For example, 1.0, 1.1 and 2.0 each map to respective CLR runtimes.
In a clear departure from this approach, Microsoft is bundling Window Communication Foundation, Windows Workflow Foundation and Window Presentation Foundation under the .NET 3.0 moniker. Unfortunately, these new platforms, while significant, really have nothing to do with a new runtime, they merely run on the 2.0 CLR, use the 2.0 compilers and will have design time support within the Visual Studio 2005 IDE.
So why use .NET 3.0 as the name? Marketing, maybe. Or maybe the team never intended to have a one-to-one FX to Runtime relationship. Some have commented that 2.x would have made sense. I can see it both ways, because by naming it 3.0, Redmond makes a clear and surgical departure from the CLR version 2.0..... That the "core" (CLR, BCL, etc) is running on the 2.0 runtime is secondary- the platform, the framework has evolved.
Platform Support
.NET Framework 2.0, as it is known today will be supported from Windows 9X through Vista and Longhorn. .NET 3.0 makes a backward compat break and is supported beginning with Vista. I am OK with this. You can't support legacy OS' for ever. This is part of the rationale for getting away from Win32, remember?
Servicing
Any parents with small children are undoubtedly familiar with a Disney show called "Higglytown Heroes". The main characters are like Russian dolls. Hollow in the center and split at the torso such that other smaller dolls can fit inside. Picture .NET 2.0 as a small Russian doll and .NET 3.0 as a larger one. Now, stick .NET 2.0 into .NET 3.0 such that .NET 3.0 now includes the .NET 2.0 doll. This is the integration plan for both framework versions. .NET 3.0 is said to be additive. That is, it is an addition to, or built on top of .NET 2.0, with no changes to the core. These additive bits inlude WCF, WWF, WPC and CardSpace and are referred to as Green Bits. Changes to .NET 2.0, specifically the runtime, will be managed via service pack level support. These defect fixes will be referred to as Red Bits.
So, we have Green Bits and Red Bits. Green Bits are additive and DO NOT make any changes to the .NET 2.0 core. The 2.0 core is safe and sound inside the .NET 3.0 doll. Should a change need to be affected to the .NET FX 2.0, Red Bits will be released INDEPENDENTLY of .NET 3.0.
.NET 3.5???
Just when we have our hot little hands on .NET 3.0, new technology additions to the platform will be ready to ship. One example might be LINQ, which will have impacts to the .NET 2.0 core. These Red Bits will be carefully reviewed to ensure that backward compatability is maintained. In other words, in making the required changes for LINQ, the existing components should not be affected.
Hope this helps.
References: Interview with Jason Zander. Somasegar's blog.