I work one day a week at Gamestop (I have a real job too, heh) and if the impressions I get from people are any indication of the DS to PSP sales here in the US then :
1.) To the uninformed the PSP is still barely maintaining it's "Oh wow, gotta have it to be cool." status but has decreased largely since its released.
2.) To the informed and those who own a psp it's either "$250.00! AND I have to buy games and expensive UMD's, AND a memory stick? The DS is only $130.00 new and it has all these sweet games coming out, 'purchases ds'....
...or ,for those who already own a PSP, "Yeah the PSP is great 'this justifies the purchase', but I don't really play it much anymore, it was cool at first but im just waiting for some better games to come out... 'sites some sweet umd movie'". I then tell them how sweet the DS is and about all its functionality and they are usually surprised but don't totally buy into it because they have probably invested $500.00 in a psp and they have to stick to their guns.
This is a hugh shift compared to what the average new/uninformed gamer thought during the psp launch window. Something like "PSP is the OwnZorz! The graphics are awesome on a hand-held! DS, was that?!?!" Heh.
Back to topic, it seems according to my observations that DS is outselling PSP by a good margin, especially in the last few months... though is falling short of complete domination like in Japan.
Anyways, I think the DS sales here in the States will somewhat mirror thise in Japan eventually (though maybe not that hugh) It's just taking longer for the PSP buzz to wear off here.