There is no doubt that Heath Ledger's Joker has a firm place in cultural history--artists such as Pure Evil have done amazing pieces on the character, while others have gone to work with Photoshop.
Here's my problem with everyone dressing up as Joker for Halloween. Once a year you get to wear anything you want, so surely it is good to push the envelope. Why would you want to wear such a predictable cotume? It is akin to finding out a woman has on the same dress as you and looks better in it!
I feel an incoming Joker costume Flickr group that features the good, the bad and the ugly. Some of the efforts featured here, for example, are pretty poor.