Well, mods in the Pc sence (additions to alter/enhace the gameplay experience) has recived a very bad reputation due to the fact that only jtags/sdks can use them, thus resulting in a lack of mass public to admire and use the work.
Now on the internet terminology tends to blur together, resulting in mod=cheat, which is not the original meaning.
Because consoles are intended to be a more cheating-free platform everyone (ms, bungie, etc.) tries as hard as they can to prevent every form of modification. It prevents a lot of cheating, but it also kills any chance of a modding community :(
Besides, changing skins is extremely difficult on a console since all of the information is read from the disc, thus the only real way is to make the console read from an image that has been modified propperly. You may have to overcome some kind of protection software too... Then, the only way for everyone else to see the skins is to transfer the files to the other consoles in the game, which halos network model probably doesnt support.
In the end, i think that mods needs to remain a quality reserved for the pc. Lets hold our thumbs for that someone ports it to pc (in a few years we may even be able to emulate a 360 environment on a pc and then we can enjoy ourselves!)