Stuart Henshall has an interesting take on Skype's move to allow PSTN dialing while sticking with a proprietary protocol over using SIP. Check it out:
Skype Softphones?" href="http://www.henshall.com/blog/archives/000900.html#comments">Unbound Spiral: 5 Million Skype Softphones?

Also, another blog from Alec Saunders has an interesting take on Skype and their use of a proprietary protocol. Here's Alec's blog entry:

"Stuart Henshall describes the impact of the PSTN interconnect that Niklas Zennstrom has been talking about. The impact is that there are suddenly 5 million softphone users who are dialing their friends at calling card rates, and urging them to get on Skype. If played correctly, it’s the next viral marketing play for Skype, and a huge problem for the upstart ITSP players like Vonage.

As the player with momentum, their strategy is obvious. Maintain momentum. Arguments from the SIP community centered on moral suasion are irrelevant."

Wow... Does Skype indeed have the momentum to skip the "SIP bandwagon" and go their own way? That remains to be seen.

I do know the main reason why Skype grew so quickly was that it allows "free calling" to anyone in your Skype buddy list, with an easy to use interface and with excellent voice quality.

I'm not so certain once Skype starts offering "pre-paid calling cards" if this same audience is willing to bite. Sure, Napster was once free and I suppose they're doing OK now that they are a paid model, however the "paid" Napster is no where near the size or market penetration that the "free" Napster was.

History could repeat itself with Skype and its impending "pay for PSTN calls" model. It should be interesting to see what happens.