Maybe it was a "1up Yours" podcast from the last couple months, but I'm pretty sure I recall hearing some strong insinuations that this beta isn't going to hit as soon as previously thought. I think May is now more likely than the remainder of this month or April.
It makes you wonder if it's going to drop with (almost) no advance notice, or if we'll get a date a month out for losers to endlessly 'refresh' their Crackdown 'Marketplace Content' section on.