In an investor Q&A from the 25th, Nintendo managing directors, asked about future internal development, noted that while overall development effort remained steady, that the company has been emphasizing Wii development over DS development by shifting developer headcount from one platform to the other.

Nintendo's reasoning for this change is likely due to DS being extremely well-supported by third parties at this stage in its life, while Wii, despite phenomenal hardware performance, is still coming into its own software-wise.

Investor Q&A (Nintendo)