I frankly think it depends on how the DS is received and whether or not Nintendo likes what it does enough to continue down that development path.
In other words, if the DS continues to be a big hit even after PSP is launched, and if Nintendo thinks the stylus control still has unused potential, we may see these things in the next Gameboy or Nintendo may even drop the Gameboy line in favour of DS (seems less likely to me than it did six months ago, but still possible).
If the DS starts to falter after PSP's launch, or Nintendo loses interest in stylus control, I think this will be the first and last we see of the system.
Could Nintendo continue DS and Gameboy lineups side-by-side? I've always been skeptical about that, but I'm beginning to think it might try. Frankly, if Nintendo does go in that direction, I will probably pick one handheld and stick with it, I don't have time for two.