It looks like the next set in the Architecture series will be 21007, Rockefeller Center.
No details yet on Lego.com - the first sets (the John Hancock Center, the Empire State Building and the Space Needle) were called the "Landmark Series" and the next ones (Fallingwater and the Guggenheim) were called the "Architect Series". Next up is supposed to be the "Discovery Series". Oddly, the Sears Tower set has disappeared from the Lego.com site, but is still there on Adam's BrickStructures page.
Edit - The Brothers-Brick came up with a better picture. Set designer Adam Reed Tucker is donating a signed copy of the set to Creations for Charity, a great charity where you can buy MOCs by some of your favorite builder and all of the proceeds go to kids. Apparently set 21007, Rockefeller Center, is going to be released in early 2011.