You can't get a blank DS Flash cart at the moment, but it's probably an eventuality. It happened with the GBA. You can get a passme that redirects the DS to a blank GBA cart and run homebrewed programs, but it's bulky, and looks flimsy.
There is something coming out called the NeoFlash, which looks legit enough, but there's some speculation as to weather it's an authentic product, or not. This is assuming that encryption and/or signature codes and the likes can be bypassed or cracked.
A handheld version would be great, but can you think of a good control setup? The DS (and most other handhelds ie PSP, Gizmondo, & Zodiac) have a D-pad (and/or analog stick) four face buttons, and two shoulder buttons. You could map it a few different ways.
You could go the standard console way and Move/strafe with the D-Pad, look with face buttons, and fire/alternate fire/dual wield with shoulders. How would you switch weapons and access terminals, though? It could be reconfigured to make it harder to vid and give you those options, I suppose.
