however there is defiantly going to be some project that is going to move to python 3.0 immediately.
you are assuming that one of the goals of the project is to be included into Fedora so Fedora can lay down requirements (like "don't use Python 3")
other systems will use Python 3 and programmers will write things in Python 3.
if Fedora wants to include these things they will need to include Python 3 or re-write them
there is another thread this week in the LWN comments where people are advocating that the distros should not modify any packages and should just ship the upstream packages unmodified. this is an example of how it's not going to be possible.