Plenty of other Macs not listed by jetfuel have had their share of nicknames. It's easy to see why, when the official names are sometimes rather uninspiring, and telling models apart can be rather painful without distinct names. For example, my iBook's official name is simply "iBook", and its long official name (used in tech support documents or while dealing with them on the phone) is "iBook (32 VRAM)". Useful, ain't it?
The different G4 towers needed some way for people to tell them apart, so now we have things like MDD (Mirrored Drive Door) and WindTunnel (thanks to the speed holes). The G5 has been affectionately called a "cheesegrater" because the front of the metal case is full of small holes.
Of course, the nickname tradition isn't something new. Older Macs had nicknames too, like those in the original LC series, which were the "pizza box" Macs.
My personal favorite name I've ever heard used to refer to a computer model was for the Blue & White G3 desktops. I don't know where it came from, and it's relatively uncommon, but it amuses me to no end to hear them referred to as "Smurf Towers". It's entirely possible that the world would be a better place if more things had playful or silly names. How could anyone possibly get angry or worked up about their Smurf Tower acting up and eating their homework or something for work? With a name like that, you just can't.