The DS is a seperate product line moreso in name than anything else.
It's a coy marketing strategy ... if DS fails, then Nintendo still has one more shot at the PSP with Game Boy Next.
If they had called it Game Boy DS or Game Boy Touch, and watched it falter, it would be a bigger blow to Nintendo. With the DS, they can sweep it under the carpet if it fails.
The GBA SP is also selling so well, that releasing another product in the Game Boy line probably wouldn't be the best idea. This way they can continue to sell the GBA SP for a little while longer.