The issue is that Skype is doing something that applications should not be doing: modifying its own application bundle. See this article on the issue of code signing, self-modifying apps and the firewall:

http://homepage.mac.com/simx/technonova/C488455530/E20071107233441/

... Leopard still uses ipfw from BSD... The System Preferences are just ooey GUI candy on top of it..

http://developer.apple.com/internet/security/securityintro.html


Please Look this site.
Title is Skype on Leopard.

http://antiqblue.exblog.jp/

regard.