Looking at market trends, 2007 was the peak year for the DS. This is significant in that most systems peak in their third year (PS2's best year was 2002, for example), whereas 2007 was the DS' fourth year. Also significant is that it's slowing down at a much more reasonable pace than most systems do. Like the PS2, which took 3 years before it really started seeing hardware sales drop by large quantities (2007 was not a banner year for the system), we will probably see something roughly like this for the DS:
2004: ~2,750,000
2005: ~11,000,000
2006: ~21,000,000
2007: ~29,000,000
2008: ~25,000,000
2009: ~20,000,000
2010: ~15,000,000
2011: ~10,000,000
2012: ~5,000,000
2013 onwards: under 5,000,000 cumulative, but close to it
For an overall number of units sold being somewhere around 140 million.
The Wii is far harder to predict, since it's only in its third year, and is experiencing such severe shortages that there's no way to even guess what its 2009 sales will be like. Until supply can meet demand effectively, we won't know the point when demand will peak.
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