Skype is hot because they have sofar done a better job than most at:

Intelligent, easy to use interfaces (compare to Gizmo for ex: and you’ll lose 50-60% of your average users to skype)

Integration with popular contact manager (Outlook is still very popular, despite the web 2.0 hype)

Offer tools for small businesses and even enterprise to handle ‘accounts’ for multiple users (security still a big issue for enterprise)

I am not a skype-pusher. I use skype, Gizmo, JaJa etc.

I just want to balance your arguments with the ‘reality’ of your average joe user. Most users could give a crap about SIP open protocol. They want to fire up an application that installs seamlessly, that looks pretty and has good voice quality (Skype has all these).
The fact that they are investing mental capital into a proprietary system is of no consequence to them, because the competition offers no better solutions when it comes to those requirements.
(Similar stories of pretty but locked in would be iTunes, and MS Outlook)

So the only way for pure SIP solutions to be adopted by large numbers of people is if they focus on GUI, UI and top-in-class tool integration.