It's a sly comment on our tendency to aestheticize Apple's branding. Like sperm, we are heading to the Egg in Cupertino. More importantly, it makes people laugh and often sparks conversations.
The other thing that I like about having a custom design is that you can quickly recognize your computer at work, in a cafe or in the TSA line.
There are a lot of other fun designs on many sites like these. You can create your own, very easily. This is how I did the "Spermintosh"
Step 1: Come up with your design
I decided upon the sperm-as-symbol after talking with some friends who were having difficulty getting pregnant. It struck me as a compelling one — biological in nature and yet universally-recognized. More importantly, it is under-used and only works in multiples. One sperm cell looks like a tadpole. The more you add, the better the design becomes.
I scoured the web for microscope images of sperm, but found that their tails were too long for a legible graphic. So, I shortened the tail enough to make them look better but still recognizable as sperm cells.
I ended up making about 10 different variations in Adobe Illustrator. Enough, that in a large mass, upon surface inspection, it feels like each one is unique.
I've included the Illustrator file for you to use, as a Creative Commons License: Attribution-NonCommercial-ShareAlike CC BY-NC-SA