You can clone in ANY pkmn game (except the counsel ones)
In emerald, you use the battle tower,
in the other adv. gen. games you clone in a way that is very similar to the original cloning glitch in r/b/y, although it is alot harder to do this time around.
Oh ya, the only way to clone in crystal is to trade w/ r/b/y and clone the old way, although I don't know if you have to turn off crystal or r/b/y...
As far as I know they removed the pc cloning glitch in he american crystal because it doesn't work for me at all, and the emerald glitch was removed in later copies of this game...