So one the WotC site they have an article about Sigil. It seems like they might plan to do more with Sigil in the future. Unlike such Design & Development interviews recently appearing on the WotC site, this one is with 2 designers who had more to do with Planescape as it's Michelle Carter and Bill Slavicsek they interview.
http://www.wizards.com/DnD/Article.aspx?x=dnd/drdd/2009October
And it turns out that some of the material from the never printed PS product City of Doors made it into the 4e DMG2.
I would like to read this, but I'm not sure why they think Design & Development articles - which are, really, advertisements promoting the sales of an upcoming product - are things they should hide behind a paywall. They should make them as accessible as possible so that casual fans read them, get excited by the thing they're describing, and go out and buy the book.