According to Bethesda's Todd Howard, the planned release date for the Dawnguard Expansion is for June 26th.
If that is the case, expect the Beta to kick of pretty soon.

I'm pleasantly surprised that he is hoping for such a quick release date (I'm a PC user, but the sooner it comes to 360, the sooner it can come to PS3 and PC). But I'm a little skeptical that they will hit the target. If they start the Beta next week, which is what I believe he said would happen, they will have about two weeks between the start of the test and the tentative release date. I don't know how long they plan to hold this beta, but two week seems like it would be on the short side for a beta test. And, correct me if I'm wrong, wouldn't it have to go through Microsoft certification after that?

It may have already been through the Microsoft certification. We have a lot of details and it's obviously almost done so they probably already did that, and if they didn't it wouldn't take long. Also, maybe there aren't many glitches they've noticed, and just need a few beta testers to find these smaller glitches, it might be 100% done on their part for all we know.

Wouldn't the final product have to be the one to go through certification? As I said, I'm a PC user, so I don't fully know the details of the process.

Yeah the game is "done" he said in the interview.

I want them to release it soon. but i hope its not buggy like fallout new vegas "Dead Money" dlc was a release

Next week is the beta

Is the 26th date the release only for X-Box?

Yes. Timed release, so xbox gets it a month before anyone else.