That's for the 10-way conference calling features. Non-Intel Core Duo Macs will still support up to 5-way conferences (depending on system requirements of course). It's the same thing on Windows. Skype and Intel made a deal to lockout non Intel processors for the 10-way conference calling feature. That includes AMD. Annoying, yes, but certainly not enough for me to really care.
What amazes me about Skype compared to iChat is it seemed to virtually require a headset. I tried talking to someone over two computers with speakers and microphones and the echos were so prominant as to sound like wavelike interference. Never had that problem on iChat.
On the other hand, with headphones on, it sounded Skype sounded crystal clear.
I've used the built-in mic and speakers on my PB for Skype without any problems - maybe it's a matter of find the right balance of volume. But I do agree that the sound with a headset is better. When I use my Logitech USB headset, it sounds like the person with whom I'm talking is in the next room - the sound is even better than a landline.