They're not having an open beta because of the absolute FLOOD of information and videos that would result online, information that they would have little or no control over.

It's a slightly riskier move to NOT have the open beta (less chance to squash bugs), but at the same time, it means they can have a "real" release and not a gradual flood up until the actual release date.

@Scott Actually, Daniel Erickson himself said that the reason an Open Beta isn't an option is because the fanbase is WAY too large for an open beta. Imagine 1.2 million players rushing to get into the open beta servers at the same time. It would be chaos.

http://www.youtube.com/watch?v=YibE7jtHC6E

Jump to 1:20 for it.

He does say there if there is a closed beta, it will be very close to release. I'd imagine a closed beta would be their form of stress testing by mass inviting, without opening the flood gates, if you will.

"Open beta" stopped being about testing a while ago. It's more become a demo than anything else. It's all another marketing scheme to let people get a taste of the release before it's actually out.
They'll most likely have a head start like Rift. That way, it staggers the crush of players into two floods instead of one, server crashing, mess. The early adopter gets names reserved and gives them ideas of how many more to expect at release.