But probably not for the reason you think. The code is ready to go. It's actually the demand causing the delay. While the public beta was supposed to go live this afternoon, the demand for the beta (which went live for TechNet/MSDN subscribers yesterday) has been overwhelming. In fact, such a high demand exists that the Windows team is beefing up the infrastructure so as not to disappoint those trying to download it.
The official message can be found here.