Will this technology work with ASP.Net based applications? If so, could we see an ASP.Net based demo? We are seriously considering this technology for our next project, but it needs to work with ASP.Net

Yes, this will work with any .NET application. What in particular are you looking to see demonstrated in ASP.NET?

"What in particular are you looking to see demonstrated in ASP.NET?" - I'll provide a couple of ideas: In one sample, a no-frills RSS feed reader ASP.NET app is the basis. The plug-ins allow viewing the RSS in different ways. Second idea: Try to boil down into the very simplest terms, a community web app that allows "applications" to be added to a page on the site. The plug-ins are a few very simplistic example "applications".

It seems the MEF works only with a host (exe, iis, etc.), what if you wanted to use the MEF in a component library DLL, like the Enterprise Library? In other words, the component library would contain base functionality but use MEF internally. Consuming applications wouldn't know that your component library used the MEF. Does that make sense?

With regards to ASP.Net, it would be nice to see an example. Using MEF can ascx usercontrols reside in plug-ins, or are we still stuck with writing server controls? (The inability to share user controls amongst different web applications always struck me as a major limiation of ASP.Net).

Good news: Maarten Balliauw has posted a MEF sample for ASP.NET MVC. Find it here: http://blog.maartenballiauw.be/post/2009/04/21/ASPNET-MVC-and-the-Managed-Extensibility-Framework-(MEF).aspx and Hamilton Verissimo has posted an MEF ASP.NET MVC sample here: http://blogs.msdn.com/hammett/archive/2009/04/23/mef-and-asp-net-mvc-sample.aspx

Is MEF also good for object collaboration or is it only a UI tool

Its a start to see an MEF example in ASP.Net MVC, but how about normal ASP.Net. I have yet to see a clear cut simple hello world example. This is kind of telling me that its not possible. ASP.Net needs a good way to easily have plug-in functionality with user controls...