Judging from the articles I've read at WOTC's D&D site, it looks like they are planning on returning to using Planescape-style old-school celestials and fiends! And also at least some of the Great Wheel planes are being used as well. Anybody got more information than I have? Here's hoping that they are going to restore the entire Great Wheel cosmology and creatures (and, dare I pray, Sigil?)
Planescape, Dungeons & Dragons, their logos, Wizards of the Coast, and the Wizards of the Coast logo are ©2008, Wizards of the Coast, a subsidiary of Hasbro Inc. and used with permission.