See the new features of .Net 3.0! .Net Framework 3.0 combines the .Net Framework 2.0 APIs with new technologies for building a broader range of applications.
That would lead one to believe that the latest version of .NET would at least incorporate some of the previous version's functionality. They don't seem to have information on the MS site about removing previous versions of .NET though.
I looked at several systems on my network that have .NET 3.0 installed and all of them still also show the previous versions in the Add / Remove Programs dialog.
I guess my first question would be, why do you want to remove these? I would be wary about removing something that is so integrated into the OS as .NET is.