Incorrect. Select customers also have had ML since day one of the first release to developers. It is called the "Apple Software Customer Seeding" program.
I don't know what the selection criteria are, but I suspect the products you own, the setting, and how you interfaced with tech support in the past plays a role. I too am curious as to what the update path is, and installs for my other Apple products. The seed docs don't say, tech support wasn't absolutely sure about it either.
All in all, it has been fun participating. I took a pass on Lion, but it did come on my wife's Air and I was not a fan of that release. Mountain Lion, on the other hand, is really, really nice.