I signed up online a couple of weeks ago and didn't hear a thing either. Had just about forgotten/given up on it...then I actually installed music beta app from the market a few days ago. Whaddaya know, within hours I got the invite for the cloud beta...coincidence? Maybe who knows.
Above poster is correct though IMHO, you're not missing alot...its pretty vanilla and clunky. Personally I like Amazon Cloud Drive better.