When you watch the dvds on the iMac even when there is a big telly and dvd player in your living room, just as a habit...
When you attach the Apple On Store (Music) Event - poster to your fridge.
When you look for digicams online, and after finding some that you like, you go to bestbuy to try them - and then order the one you like most from online.
When you first check the mac board and then go to get the morning coffee, and before going out you check the temperatrue on your menubar.
When you feel bored when you don't find any nice arguments in one day on a board...