(I thought this was already true, but apparently not—you could refuse to show ID and would be subject to additional searches but would be allowed to pass the security checkpoint.)
However, if you say you forgot your ID, you can enter (additional searches and interviews may apply).
I like this comment from Chris Soghoian at http://news.cnet.com/8301-13739_3-9962760-46.html:
TSA's new rules only protect us from a non-existent breed of terrorists who are unable to lie.
(One of the comments to this post disagreed with Soghoian's interpretation of the rule.)