What I'm asking when will be the next time the DS falls to second in monthly sales in the US or weekly sales in Japan. Honestly I don't see it happening in either market this year. The Wii is probably the only system that has the ability to perform the tast in the near future but I don't see supply being strong enough. Now if you really want to grasp for straws you might say the 360 could do it September, Halo 3 month. Very unlikely though. Sure it will give the 360 a nice spike in sales, but it would have to double monthly sales to match what the DS is doing and that's not going to happen.
If I had to guess I think it happens early 2008 in the US when supply situations with the Wii are pretty much solved. In Japan it might not happen until mid to late 2008 just because DS sales are so insane week to week right now. Heck Japan has barely solved DS shortages, so Wii shortages are going to be prevelent for a while.