Nethaera steps in to answer his question, and shed some candlelight. Blizzard pulls its beta testers from "many places." Some folks win a contest, others are random, while still others are the result of specific targeting -- they want that particular demographic of player represented in the testing. They don't pick only the most experienced players, which makes sense when you consider the game's for everyone and not just the elitest of the elite.
Neth wraps up her posting by saying Blizzard's not yet determined if there'll be some kind of application for the beta, and that it'll all depend on what the developers want when that time comes around. I'd guess they've done some thinking on this subject. But in some vocabularies, alpha testing just means "a development phase." We're probably still a little ways off from the closed beta.
Edit: Added a link to the thread in question, in addition to the Source link below.