Recently I've read a lot of posts that say that the Nintendo DS is an experiment and that Nintendo seems ambivalent in regards to having their future portables being catridge based or the DS's new media type. It sounds like Nintendo is leaving it completely up to the consumers to decide through sales what the future will hold.
If this is the case, the Nintendo DS has a very uphill battle if it hopes to be that new Nintendo medium. It has to use its two screens incredibly and have graphics to boot so it can even compete with the huge following of the preestablished GBA (and its ancestors). Along with this as nolimit said, Nintendo needs to push this thing. Probably harder than the GBA because it does not have the backwards compatiblity that endeared so many to the GBA.
The new DS though has some serious advantages. The new media has been compared to disc media in that it is easily writable and that will hopefully mean a proliferation (Playstation style) of games. We also know that the media has much more memory than the GBA catridges meaning longer and nicer games (potentially). Two screens by themselves aren't that great, but coupled with the above, it can only be a positive.
Writing this just now, I realized that if Nintendo wanted to put out new portable hardware as the successor to the Game Boy that wasn't backward compatible without the option of another backward compatible system, we'd lynch them... pretty hard. That attitude, making every thing backward compatible, is eventually going to limit Nintendo. In fact, now I'm confident that the DS is direct competition for the PSP.