Mario Party DS preview at IGN.

"Let's just get this out of the way: Mario Party DS will not have online multiplayer. Now, before you start having flashbacks to Mario Party Advance, a game many generally consider the weakest of the Party bunch due to its lack of multiplayer fun, the series return to a Nintendo handheld does include four-player Party Mode, where you compete with and/or against your friends. Phew. And while you can't play online, the Multiplayer mode allows you to wirelessly host a four-player Party or Minigame mode session (or a two-player Puzzle mode game) via a single cart."

Full Source: IGN