Second, my anecdote is completely unscientific, but merely an observation--and an interesting one at that.
I have two kids. A four year old daughter, and an 11 month old son. My daughter's room is strewn about with all sorts of "girl stuff"--i.e. princess pictures, girl toys, headbands, barrettes, ya know... "girl stuff"... she also has an assortment of matchbox cars (more on that later), megablocks (huge lego-like blocks), and these fisher-price trio blocks (these are pink, its the "girl edition")...
So anyway, my son crawls into his sister's room constantly.. wanting to play with her toys, he's always going for the cars, the blocks, the mega blocks, or the trio toys (they're not quite like legos, but you build stuff with 'em)... never mind there's a tea set, a metric ton of dolls, my little pony, princess wands, play jewelery, tiaras, etc... he ALWAYS goes for the blocks or the mega blocks, or trio toys... the mega blocks are not pink, and the wooden blocks are the same wooden blocks you and i grew up with... okay, we have not taught my son that a specific toy is a girl toy, or a boy toy... he just goes for the toys he likes... and they just happen to the toys boys like...
about the matchbox cars, my daughter loved the little cars when she was a baby, and she still plays with them.. funny thing, though, she doesn't like them as much as her "girl toys.."
i dunno.. seems like its a pretty natural inclination, unless it is "skewed" by the likes of this idiotic mother we just read about...
Granted, he had a lot of Matchbox and Hot Wheels cars that were mine and several that were his older brother's (who was more into Legos than cars), but he had other toys, too. This obsession or passion for cars is all him. If you ask him what he wants for his birthday, it's "Cars!" If you have to bribe him with something, it's a car. Instead of talking about what he wants to be when he grows up, he talks about what cars he's going to have (3 limos and a Hummer. Sometimes 3 Hummers and a limo). He has, literally, over 300 Hot Wheels cars at our house and probably that many at his grandparents, who often keep him. He can tell you what every single one of them is, and usually where it came from. He can tell the make of every car on the road. At first, he would pick out the emblems, but now it's headlights and taillights and rooflines and many models as well as makes.
Of course, he also puts on his mother's fuzzy pink slippers and walks around in them, but I'm going to leave that one alone for now; he's just trying to be funny.