According to several security websites, upgrading to Apple's newest operating system, Leopard OS X 10.5, shuts off some basic security controls. Namely, it appears that upon installation, the default firewall rule in Leopard is "Accept all incoming connections".
This setting is highly risk since it exposes the computer to a possible network-based attack. It is recommended that the firewall rules are reviewed on all Macintoshes with Leopard installed. The firewall should be set to a more restrictive setting such as "Block all incoming connections" or "Only allow connections to these applications / services"
To do so in Leopard, go to System Preferences > Security > Firewall, change the setting to either of the aforementioned, and then save all changes.