The ASP.NET 3.5 Extensions Preview (which includes microsoft's new MVC framework) should be release today on ASP.NET. It was due to ship earlier in the week but due to a bug got delayed. However, the team behind it have been working over the weekend to get it out. I can't wait to get my hands on it.
I've used ASP.NET for development even though I know about the other approaches (Ruby on Rails, Castle etc...) simply because I make a living out of coding and most clients want the big corporate supplied platform. Going into a company and trying to sell Ruby on Rails can be quite difficult and there simply aren't the opportunities / jobs about to justify switching to it (although I can understand people using it on private projects and startups).
Some question whether Microsoft should be producing a copy or clone of what existing open source projects already do. Well, I'm glad they are doing it!
Love em or hate em, Microsoft make some great development tools and platforms / frameworks and using these is how I make a living.
What I think the ASP.NET MVC framework will do is validate the approach (if it needed it) and also allow it to be used in corporate / enterprise environments - i.e. it will become mainstream and less time will have to be spent selling it to people who really don't know the difference between a MVC and an MCP but don't want to risk their position by allowing some developer or consultant to lead them down the road to a minority used platform.
ScottGu has made some posts about what the MVC framework is and getting started with it which are a great read:
ASP.NET MVC Framework
ASP.NET MVC Framework (Part 1)
ASP.NET MVC Framework (Part 2): URL Routing
ASP.NET MVC Framework (Part 3): Passing ViewData from Controllers to Views
ASP.NET MVC Framework (Part 4): Handling Form Edit and Post Scenarios
Also, some other blog posts provide more detailed information about various aspects:
TDD and Dependency Injection with ASP.NET MVC
ASP.NET MVC Preview: Using The MVC UI Helpers
I can't wait to use this and already have some apps planned to use it ...