I don't think they'll merge the two variants. Rather, I think after there's a transition to touchscreens for most/all Apple devices, they'll just make the iPhone OS default and run older apps in a "mouse/keyboard" compatibility mode. Of course, this is probably quite a few years down the line. I'm sure there will also be some back and forth in features between the desktop OS and the mobile OS (as you're already seeing with the new auto-correct features in Snow Leopard).

They should pull a Centrino. Just acknowledge that their latest sucks and go back and optimize and refresh XP. Use what they learned from Vista's development and not make the same mistakes.