I don't think there will be an open beta, but I do think that it will end up similar to the starcraft 2 beta in that it was extremely easy to get in towards the last few months.
It would be awesome though if the beta ends up going open, but I doubt it will start that way.
I'm hoping it ends up like SC2, where if you pre-ordered the game you'd get a beta key.
Key words being "If all goes well." Blizz is notorious for taking their sweet time to iron out even the littlest bugs. A strong launch with few glitches is HUGE in today's market, and if they rush it out to get holiday sales instead of doing a thorough beta, people will start complaining that it was rushed and more people should have had the chance to test it.
In short, if there are enough bugs to delay, be ready to play something else into 2012.
A few glitches is not HUGE in today's market, it was huge 20 years ago, its actually the opposite, thanks to the internet, patching is being used as a crux to enable games to be rushed to release because of pressure applied by parent companies or investors. Fallout 3 is one example, off the top of my head of a game that was released too early, and still has substantial bugs (like alt-tab full screen mode) that will likely never get fixed.
Blizzard is incredibly lucky to be able to create and release games the way they do with no fixed deadlines, considering they're in the private sector. Most companies are not that lucky, and it shows in there games.