There are a lot of people out there having problems with Skype on Leopard. The issue is that if you have your firewall switched on, then Skype simply crashes on launch.
There are a number of workarounds for this but most of them involve switching off the firewall, which is something that most people dont want to do.
Ive came across a couple by myself just from my own trial and error. The first is to reinstall skype, it will then work when you launch it but after closing it, it stops working and needs to be reinstalled again, which obviously can become a hassle.
To get around this - just keep the .dmg file for Skype and run the application from there, from here it runs and you save at least a couple of clicks from reinstalling it into the Applications folder.
Skype is currently working on a fix.