I know DSNES is out, but it is not perfect yet, and not too many games run full speed yet.

DOSBox requires substantially more computing (particularly processor) power than the original systems, and is greatly affected by what software the user is running in the emulated system at the time.

The DS uses two ARM processors:

ARM7TDMI(-S), which runs 15 MIPS* @ 33 MHz
and
ARM946E-S, which runs 68 MIPS @ 67 MHz

and only has 4 MB of RAM.

Now that hardware has to emulate at least a Intel 386, which runs 11.4 MIPS @ 33 Mhz. At the same time, it has to emulate the Video GPU, which Dosbox emulates the S3 Trio 64 card. The Trio was named such due to the fact that it had 3 Proccessors in one. (The graphics core, RAMDAC and clock generator) So, you have to take that into account.

Now don't forget the Sound Processor. Dosbox eumlates a Sound BLaster 16, which is powered by The Yamaha YMF262 OPL3 chip.

And, finally all of the other remaining hardware has to be emulated. (The Motherboard's Northbridge and Southbridge chips, The Memory controller, the Serial and Parallel controllers, etc.)

On top of all of that, Then the DS would have to take it's 4Mb of Ram, to hold the Emulator program, the CPU's RAM, the GPU's RAM, and the SPU's RAM. Then, if there is any memory left over, it can be used as XMS/EMS RAM for Dos.

I honestly don't think the DS is up to the task.

The GP2X which uses two 200MHz ARM9 processors, as well as 64MB of RAM, can only run Dosbox with limited success. (Though, it kicks complete ass on any game console from the the 16/24 Bit generation and back. aka Atari 2600 thorugh the SNES, GENESIS, and Neo Geo.)

*"Million Instructions Per Second"

By far, since that is the main "unspoken" reason why it was created. It works great, and you don't need to hack the bios or anything else to run them, just throw them on a memory card and you're off.

Here's a list of emulators available for the GP2X:

http://wiki.gp2x.org/wiki/Software_Directory#Emulators