rumor control
Just so everyone knows, when I asked if and when a 4th edition was coming out at the D&D Q&A seminar at Gencon, they responded that there were no plans for such a product anytime soon. WOTC (both the business folks and the R&D folks) feels that 3E has plenty of room to grow before stagnating and you wouldn't see such a product for at least another 6 or 7 years.
As Morrus said, they are thinking about a 3.1 version of the rules, with some errata applied, minor changes, and clarifications.
Another thing they're thinking about is a leather-bound annotated version of the Player's Handbook. The annotations would be comments in sidebars by the designers, explaining various design decisions, etc. I assume if there is a 3.1, this would be after that, but no one really knows because they haven't made up their minds about any of it.