According to E3 it's the 12th for the beta and 26th for the actual date!

12th for the beta? What?

No way...
They really need to release the beta sooner for better results.

I think they are pretty confident that there are no gamebreaking bugs, like the 1.2 patch.

Todd Howard said that they will only release the DLC on the 26 if the beta goes as well as they hope. So if no one finds any game breaking bugs in the beta then it is safe to assume that it will be released on the 26. But if there is a game breaking bug then the DLC will most likely get delayed.

In other words, it may go fairly smoothly without too many serious bugs like patch 1.5, or it may take much longer than expected due to serious bugs, like patch 1.6. Hopefully it will go as hoped and they will be able to release it on the date they want it to, but it's likely that it will get delayed because of new game-breaking bugs. Of course that depends on how game-breaking it is and how many people will be there to debug it. It will likely be a larger team than the patches due to the sheer size of the DLC, which would likely mean faster fixes.

God im hoping bethesda gives me the dawnguard beta im just so excited, if i do get it i will be so happy, i'll probably cry haha nah but im hoping.

Todd Howard: "It's coming out on Xbox" *cha-ching*

Interviewer: "When is it coming out for other systems?"

Todd Howard: "You mean PC? I don't know, but the 26th for Xbox" *cha-ching*

Interviewer: "Is it even coming out for Ps3"

Todd Howard: "Wat?" *cha-ching*