When you keep your older Macs around, even if you hardly ever take them for a spin anymore, once you've upgraded just because you can't bear the thought of selling such a dear friend.
When your friends are impressed with the Mac network you've set up with all of your Macs that you can't bear to get rid of.
When you can wirelessly connect to the Internet and to the various Macs around your place as well as bluetooth to your pda and mobile phone and it's still not enough connectivity for you.
Oh, and when you keep a not-so-nice form letter on your .mac e-mail account that you send off to various idiot-run companies from time to time reminding them that this is the 21st century when you discover that their products STILL aren't Mac compatible.