If rumours circulating the Internet are to be believed Nintendo has started to bring the ban hammer to people who have used the Action Reply to enable some specific cheats on their Pokemon Diamond and Pearl games. So far using the codes "National-Dex (After talking with Game Director)", "Catch All 493 Pokemon" and "All Shiny" has allegedly resulted in players getting banned. Good thing too, we don't want Pokemon to end up as another crapstorm of hackers like in MKDS..
Source: Wii60.com