This from WotC's PR department:
You may have been noticing that questions regarding DUNGEONS & DRAGONS(R) v.3.5 backward compatibility have been popping up in the roleplaying game community.
Coinciding with the July release of the DUNGEONS & DRAGONS v.3.5 core rulebooks, Wizards of the Coast will offer a free backward-compatibility guide to enable players to use their existing DUNGEONS & DRAGONS accessory titles with the new core rulebooks. The free guide includes revision notes for the EPIC LEVEL HANDBOOK, FIEND FOLIO, MANUAL OF THE PLANES, DEITIES AND DEMIGODS, and MONSTER MANUAL II. It also contains general advice for use with any third edition D&D(R) supplement and makes upgrading to D&D v.3.5 both easy and fun. Players will be able to download the guide from
www.wizards/dnd.
The new v.3.5 core rulebooks represent the best-playing DUNGEONS & DRAGONS experience yet and, with backward compatibility, players will be able to take advantage of all that D&D v.3.5 has to offer without hesitation. Carefully designed to build on and expand players' options, v.3.5 will not make key game accessory and supplement titles obsolete.