I don't know how many of you have a subscription to kobold quarterly, but there was an interview with the 4e PHB design team in it. About halfway through a question is asked:
KQ: I'm very excited to hear about the Manual of the Planes. I guess I'm hoping that some Planescape elements will survive into the new cosmology.
James Wyatt: I'm working on the sections on Sigil right now, and In the Cage is open on my lap. Hurray, confirmation that Planescape survives into 4e!
Okay, so it's not proof of the three setting books, but at least they're checking their sources and making an attempt at it.
Reeeeally ....
I think I'll have to get a 'scrip to kobold quarterly.