The more I think about it, the more certain I am that it's coming this year. The 3DS would do a lot better in sales launching in the 4th quarter than in the 1st, and I don't think it would have any real effect on DS sales. Most of the people who would buy a DS this holiday season will do so regardless of whether or not the 3DS is there; there isn't much overlap between people who waited 6 years to buy a DS and people who will buy a 3DS at launch.
The 3DS will launch on November 21st, 2010. It was mentioned on RFN, and I saw the pattern prior to that: Nintendo launches hardware the Sunday before Thanksgiving. The GameCube, DS, and Wii all launched on that day, and I don't see any reason that will change. It also has the side benefit of being 6 years to the day after the launch of the original DS.