This is a silly Skype Upgrade bug. On a PowerPC when you've got Skype v.2.x and you (automatically or not) look for updates. It will find you an update, and it will tell you it's the latest 2.8 version, but in fact it will download the latest Intel only version. Which of course won't work on a PowerPC.
What you do is to go to the Skype web site, click on Get Skype - Mac, then you stay on top or go all the way down on the Mac page were you find Download Skype 2.8 for Mac and click on it, and then you download v. 2.8.0.866 and install it.
Or directly: http://www.skype.com/intl/en/get-skype/on-your-computer/macosx/2-8
Then you open up Skype and go to Preferences (Menu - Skype - Preferences) were you uncheck "Check for updates automatically".
At least I've got no problems and I regularly use Skype on an iBook, much much better interface than the 5.x version that I've got on Windows. I use an USB headset to get a good sound.