Coding's not that different, really, and done in the same languages that run on PCs (IE, something compiled down to machine code), but Gamebryo implements it's own scripting language which is cross-platform. If bethesda and MS wanted to, they could fairly easily get mods onto the consoles, they'd just have difficulty running them.
Aside from that, for most modding the game binaries don't need to be touched at all (exception to this is things that use a script extender). Things like the GECK tool are just editing game resources and scripts, which are pretty much the same for all versions of the game. As has been said, the big reasons you won't see mods on the consoles are that supporting people that break their game with mods on a console would be a pain in the butt and that the console manufacturers won't allow it. I know that some FPS games on consoles have editors, but they're just level editors. The modding that is done in games like Morrowind, Oblivion, and Fallout 3 is much, MUCH more extensive than that.
As far as mod tools that actually run on consoles, no way. The GECK is way too complex to use with a control pad (I'd rather mow the lawn with finger nail clippers), and most modders use a bunch of other PC software in mod creation (graphics and sound tools mostly, but also a lot more...especially in the area of custom apps for doing things with the game resources that the GECK doesn't do). It just wouldn't work.