Erm. The DS is selling so well here because the PSP isn't out. And, most people in the US don't know it exists anyway. We'll see the sales trends at the end of the year when both systems have been out for a while.
It's also worth noting that in Japan the PSP is in a better position now than DS, but in America when the PSP comes out the DS will be in a better position. Right now in Japan the DS and PSP are both out, and neither systems really has any games worth having. PSP has Ridge Racers which has shiny graphics and fun, but shallow gameplay. DS has Mario 64 which is a great game but it's one everyone's played and the lack of analog doesn't really help much. So at the moment, since games are pretty bad for both, the only thing to go upon is hardware. The DS's advantages are it's two screens, touch screen, microphone, high quality speakers, better craftsmanship, and better battery life. The PSP's strengths are its more powerful hardware, larger storage medium, multimedia playback, better screen, and analog disc. So in Japan, gamers will be taking a risk to invest in the DS since its strengths are mainly innovations that may or may not catch on. With the PSP they will be safer because its advantages are purely technical, it's just a better GBA.
In America, when PSP is released it won't be even as far as software goes. The PSP will just be coming out with a few launch games (most Japanese games won't find their way over here anyway) and PS2 ports. However at the same time DS will be seeing the influx of it's second wave of games, such as Mario Kart, Metroid Hunters, Animal Crossing, and so forth, all of which will likely support online play and will be released sometime after E3 and before the end of summer, hopefully. And by the time the PSP even comes out in March or whenever, the DS will already have a significant lead. And heck, the DS will probably see its price drop this holiday season. With both a low priced DS and Xbox2 out this holiday, the PSP won't seem as great to customers. For half the price they can get the DS, or for a little more than can pick up a brand new console. And I don't know if even Sony has the balls to drop the PSP's price so soon. Either way they are going to lose money. Keep the price up at around 200 and lose some sales to MS and Nintendo. Drop the price and see those extra sales negated by the fact that they are losing even more money. And not only that, by the time the PSP is out the DS will be steamrolling out of factories. The PSP is still struggling to be produced (and they are rather fragile and shoddily made anyway) and that just for when Japan is the only source of demand. Sony has a lot of advantages with the hardware and coolness, but they are paying a steep price.
And before you scream about how the DS is selling better in Japan, yeah it is, but it is a Nintendo handheld and taking that into account the PSP is doing far better than expected. Yeah, it's not beating DS, but who expected that? I sure didn't. Many journalists are quick to proclaim their predictions and deduce about how so and so is doomed because the other company has such and such, but these people fail to look at the entire picture and just generally be smart.