This feature doesn't really impact a gamer like me who simply wants the DS for a pure gaming machine. I'm excited about the games being developed for two screens and how the touch screen will change my gaming experience, especially in "Metroid" games.
However, for many connected gamers, this opens up the possibility of XboxLIVE-esque head-to-head battles with chat. Pretty cool, if you ask me.
I wonder if down the road we'll see a standard developed so that I could play an online game on my DS against one gamer on XboxLIVE and another online via the PS2 and/or PSP.
As long as this feature on the DS doesn't hurt the price, I don't think it'll hurt sales at all and only stands to increase them.