Announcement note at Scoble's blog
Out probably by the fall, *assuming* a short beta. Although I can't believe there'll be a short beta of something like IE: a better guess (ITHBO*) is second quarter next year before we see an actual released product.
UPDATE: XP SP2 only, apparently.
At the same time, with this type of product, launched mainly for security reasons (and other than finally implementing tabs, why else?), a common attitude to have is "why wait? Worst case scenario you are usually no worse off than if you didn't have it..." The perfect example of this is the Beta AntiSpyware product from MS. Yes, it's beta, and from MS of all companies, but I use it, and I tell others to use it. It's also the case with mediocre (but free, and included) implementations
However, there is a downside to this: a false sense of security. If you find yourself clicking on dubious links *more* or taking more chances because you think you're protected, then you're potentially worse off if the beta doesn't cover you where you think it does.
So my recommendation (in the AntiSpyware specific case) is to use it, but don't lower your shields and don't stop surfing responsibly and carefully. In other words, install it, but act like you haven't. On IE7, I'll reserve my judgement: if the beta provides enough protection benefits to overcome the inherent risk of running beta code, then yes, I'll recommend it.
Wasn't it just a couple of weeks ago that MS said "no how, no way!" on a new IE before Longhorn? In fact, wasn't there not even an IE team until recently?
(*) In This Humble Blogger's Opinion