Apple isn't putting iOS on the desktop. It wouldn't look right, feel right or work right. They will however be incorporating some iOS ideas into OS X. I really do believe that they'll do it with a careful and steady hand.
I should disclose that while I love OS X on my desktop, I have the robot in my phone.
EDIT: Added a sentence for clarification of my concerns.
As far as convergence goes, what I'm starting to wonder, is if it wouldn't make more sense to add a docking solution to iOS instead. After all, Moore's law is bringing desktop-class power to the iPad far faster and more convincingly than any case for iOS style interaction on the desktop is being made.
So the question then arises, whether it would make more sense to support a docked iOS 'mode' -- allowing use of desktop-style app UI and interaction (keyboard/mouse) in native iOS apps -- or just run OS X virtualized when docked (with some shared file bucket between the OSes)?
I guarantee there're are lot of other improvements to be seen, come summer.