As I say every week, The DS has mostly saturated it's traditional market. This is to say, kids. It is very rare for me to find one of my students who doesn't own a DS. When i asked what kids wanted for Christmas 2 years ago, everyone wanted a DS. This yeah when I asked the kids wanted Wii or PSP or more commonly just DS games. Very few kids wanted a DS, the reason for this is that they already have one (or two in some cases). The kids market is tapped. All that's left is to keep making them for the few holdouts or to replace broken units.
But, it is still selling 50k a week (numbers all systems other than Wii and DS can only dream of). The reason for this is that it is doing a good job of hitting markets that are not Nintendo strongholds. They are selling to older people, office ladies, hardcore gamers who have shunned portables in the past and people in their late twenties/early thirties who loved games in the famicom days but havn't played them since they "grew up". And there is still plenty of room for growth in these areas.
PSP is popular for high school age kids (mostly boys, some girls). They want to appear cool and they get the more hardcore looking system. Some of these kids pirate games (there are TONS of magazines that teach them how) others are buying New copies of Monster Hunter 2 (used ones are only slightly cheaper) and maybe a used copy of FFVIII:CC (used ones are way cheaper, because they are so many of them). As the little kids grow up, they will probably buy a PSP as well. There is still tremendous room for PSP growth in the school age market. I think PSP will continue to sell well for quite a while.
I personally don't see the PSP being used as a media player very much but it seems people are buying it for that as well.
50k seems small compared to the numbers DS used to sell, but when you consider that many of them are probably first time game buyers or people who haven't bought games in decades, then that number looks a bit brighter.