Thought exercise then, I would maintain a practical use of it would probably not arise
The true emulation is that of the transistors upwards which
has been done and that would be painfully slow even on a modern PC I figure. However the DS has an ARM9 which is functionally identical to an ARM7 (albeit slightly faster and with a few extra instructions) which you can see about
http://imrannazar.com/ARM-Opcode-Map and
http://nocash.emubas...htm#cpuoverview meaning you can go in for the lesser, but still very effective, type of emulation of just translating instructions and maybe modelling a few things that do not exist. This carries on a fair bit and in many ways I would call the DS a beefed up GBA with fewer restrictions: some of the quirks of DMA on the GBA do not really exist on the DS among other things.
The DS one is higher clocked but clock alone does not play the greatest role as you also have vblank and hblank to contend with may make things easier or may make them harder- many functions are tied to vblank interrupts so having a game run too fast might not be the biggest problem but unlike PCs and to a lesser extent the consoles they can effectively rely on a single relatively stable clock. Race conditions... purely by virtue of C probably being the language of choice (it was available and quite viable but I have yet to fully confirm it across many games) and the compilers not being terribly odd as far as doing crazy compiler things race conditions are probably not that big an issue but I do want to note it being a potential one.
The cart itself- others mentioned the GBA cart is mapped to memory (08000000-09FFFFFF with mirrors of variable priority at 0A000000 and 0C000000) where the DS one is not mapped at all although the 08 mirror is still available in DS mode. For my money this is the hardest problem to solve and this is big and why supercards on the GBA were less than stellar even after speedpatches which nothing else this side of the switch from the GBC needed.
The BIOS functions- several have different mappings and if you prefer some of the GBA ones are now available as extra features on the DS hardware. The audio stuff might be troublesome though.
I have probably said as such in the past but I reckon a combination of application specific emulation (been seen at various points in the past) and either a static reinterpreter (automated or hand guided I have not decided upon) or maybe a dynamic one might be able to be used to allow a few games coded without relying too deeply on the specifics of the hardware. Personally I would not put the effort in and instead spend the time designing a proper GBA flash cart- the GBA and DS count as embedded hardware in my book and with some of the fairly easy to use programmable chips and/or NOR or RAM interfaces I doubt those capable of doing any damage here would have a hard time putting a halfway functional GBA cart together at least*. Likewise if people are already making clone GBA type devices I would look there for some would be hardware (clone NES devices have long existed and many of the others are looking like they might have something at some point in the not too distant future if they have not already got something halfway there).
*NES mappers, SNES special chips and GBC bankswitching make life a slight bit more difficult but the GBA was pretty much easy street. Sure trying to get a full soft reset, cheat engine, fancy UI and more working would be harder.
This is turning into a repeat of conversations I have had on IRC on the matter so much like there I will say if someone does well on one of these then they will get a drink out of me should we meet but I hold out no hope of it going terribly far/place hope of being able to do it all on the cheap as it were. Purely as a "is it worth the time as far as creating a functional piece of code?" then no as it is available with not a lot of effort on hardware and emulation on other readily available handhelds and there are still emulators, ports, hacks and remakes of things that could do with some work (the DS emulation of the PCE/TG16 is next to non-existent and there were several notable games there). It was probably already clear but if doing it for intellectual curiosity or something similar then carry on as I am sure it would be fascinating and cause you to learn a lot.
