Nintendo has a massive year with the DS in 2006 in Japan. 2007? Less massive, but huge. And last year? Not huge, but very, very big.
French site NintendojoFR crunched last year's Media Create sales numbers. Let's have a gander:
2007 sales: 7,167,372 units
2008 sales: 3,946,785 units
The introduction of the DSi makes much more sense now, no? Keep in mind, however, these are not official sales figures from Nintendo. For comparison's sake, here's the PSP sales data:
2007 sales: 3,069,465 units
2008 sales: 3,693,026 units
More charts in the link below.