If the gossip circuit is to be believed, Nintendo may be weeding out Pokemon Diamond / Pearl players that are using Action Replay DS codes to beef up their Pokemon. While it may be true that code sniffers may have difficulty in detecting some Action Replay codes, Nintendo managed to sniff out the following:

• National-Dex (After talking with Game Director)
• Catching All 493 Pokemon
• All Shiny

According to floyd of Wii60 forums, using the codes above would block you from getting online. We don't dare try it ourselves to verify the rumors, but you can do so yourselves AT YOUR OWN RISK.

Should the rumors be true, then it looks like Nintendo is taking some actions regarding online players getting undue advantage over those who put in real work in improving their Pokemon stats. Well, fair play is always good, don't you think?