Here's some food for thought: what if the DS didn't have a four-month head-start ahead of the PSP? Both systems have had at least 22 months of sales in America. Looking at cumulative sales, this chart from VGcharts.org
shows that the PSP has sold more
than the DS in their first 22 months of sales. Certainly, Japan is a completely different story
... but does that really matter for gamers in the US? Americans love their PSP ... even if the video gaming press doesn't want to admit it.