Actually, OS X has always had its firewall off by default: this isn’t new to Leopard. Macs don’t seem to be a target for intrusion attempts in the same way Windows machines are. I suspect that is only slightly the result of any kind of improved security. Mostly, it is a result of the bad guys wanting to get the best bang for their trojan/worm/virus buck. With fewer than 1 out of 10 machines out there being Macintoshs, you can see why they put their money into Windows.

I have a hardware firewall, so for the moment I’m not really worrying too much about running a firewall on my Mac. I’ll investigate it at some point, though.