Protocol is closed and little progress has been made for reverse-engineering -- which I think would also be illegal, since the data is encrypted and then you're at odds with the DMCA (IANAL btw -- the DMCA grants a safe harbor for reverse engineering, but that hasn't worked out so well for DRM).
To my knowledge there is no alternative Skype client, not even for the IM part -- Pidgin has integration with the official Skype client and piggybacks on top of it. If you don't have the Skype client installed and running, Pidgin doesn't work.
P2P in this particular case is really not the freedom-giver technology that allows you to workaround Skype's servers. If Skype wishes to serve adds to you, or to track you, there's really not much you can do about it.
which I think would also be illegal, since the data is encrypted and then you're at odds with the DMCA (IANAL btw -- the DMCA grants a safe harbor for reverse engineering, but that hasn't worked out so well for DRM)
I believe the DMCA outlaws circumvention of copyright enforcement mechanisms, not encryption. I don't see why the Skype protocol would have any such mechanism.
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.
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.
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.