Assuming that Nintendo doesn't want to launch the Wii on the same week as DS games (they wouldn't want to crowd themselves), a recently released schedule for Q4 2006 shows a new DS game released by Nintendo themselves every Monday...with the exception of the two Mondays immediately after the PS3 launch, Nov. 20 and Nov. 27 (which surprises me), and the first one in Q4, Oct. 2.
Assuming Nintendo launches before the PS3, as they keep telling us they will, people seem to be betting on Oct. 2.
Though Nov. 20, one of the other "blank" days, is Mexico's Revolution Anniversary. Which would be neat, if we lived in Mexico.