The 400mHz DStwo is known to be somewhat hamstrung by the speed of data going from a cartridge to the DS system.
Now, I could be wrong on this, but word is the 3DS has an even slower data transmission speed and would thus greatly limit the kind of graphics/data that can be pushed to the 3ds screen. Probably not worth it for the kind of things a 1Ghz chip would be expected to do by homebrew enthusiasts.
Just like people initially said that the 3DS uses the same communication protocol as the DS because the pins are in the same place... that's something I wouldn't trust without any science behind it (3dbrew, etc.).