I'm not an expert on how the consoles operate online, however I'm under the impression that each console has a proprietary online network.
Essentially it will divide us all into three groups PS2, X-Box, or Gamecube online players. I'm personally going to get PS2 a couple months before it's release so it should be nice and cheap. However that certainly is a major drawback.
Let's just say for example most people own a PS2 because it came out before the other 2 consoles. Just for argument let's say most people like to buy their games for PS2 as well. That's fine until the people that only have Gamecube and X-Box get online and see there's hardly anyone on.
"Damn... Sorry guys. We didn't think everyone would pick a popular gaming console and leave the others out."
-Blizzard Entertainment after the release of StarCraft: Ghost