[huetsch]

I hacked on Pidgin a while back and people talked about the Skype plugin a bit. From my understanding, the issue is that the Skype code is very obfuscated and reverse engineering the protocol would be very difficult - apparently, no one with the ability and time to do that has stepped up thus far.

[ldite]

This is a fascinating analysis of Skype from a few years back:

http://recon.cx/en/f/vskype-part1.pdf

http://recon.cx/en/f/vskype-part2.pdf

Looking at how complex it is, and considering that Skype probably have more obfuscation and crypto ready to roll out if anyone cracks it, I can see why nobody's implemented a compatible client:

[stcredzero]

Security by obscurity works, if you're willing to respond with resources, and the economics do not motivate an overwhelming horde of opponents. There are definite economic reasons why Skype can make it work with just software and Sony can't even with help in hardware.

[notJim]

Another reason not to implement a compatible client is there's little need. Skype exists on the 3 major platforms (Windows/OS X/Linux), and what advertisements exist are pretty unobtrusive. If Skype where to get more aggressive with their advertising, or the client quality became unbearable, I suspect there would be more effort to develop competing clients.

Edit: this is pointed out in the linked slides.