About all we really know for sure is, the Setup Guide for the new models includes instructions for use with Time Machine. See page 13 in Airport Extreme Setup Guide.
At last check, however, Apple hasn't updated any of the support articles or Help to reflect that -- they still say either that TM can't back up that way, or that it's not supported. We've asked for clarification, but so far . . . nothing.
One of our hardware gurus, LaPastenague, reports that the new Time Capsule has new electronics inside, and that the new Airport Extreme has what appears to be the same (earlier models had different hardware from earlier Time Capsules, possibly explaining why it was often unreliable).
How's that for a definitive answer?