Don't know if this has been posted already ...
MKD has been out here in the UK for about a month and someone's already found a way of hacking into the Konquest mode to play the characters in arcade mode. I can't verify the Action Replay, Gameshark or AR Max codes, but the Xploder ones work fine (without a modchipped console). I've put up a page on my site with details and screenshots ...
http://www.angelfire.com/sd/scaifsal/mk/mkdmod/mkdmod.html
The link to the original website (European Code Base) which devised the codes is at the bottom of the page.