Same thing (I think---it is not clear from the email I got) happened to the Anatomy class I signed up for. That at least had a date (March sometime; I forget) assigned to it. Sort of makes you wonder what is going on behind the curtains?
Based on some hiring e-mails I've seen it sounds like Daphne Koller and friends are trying to spin this off into a commercial venture (at least the means to offer such courses online). So it sounds like either a) they've gotten so much interest they're having trouble scaling (hard to believe) or b) they're held up trying to sort out ownership and terms of a partnership with Stanford.