Meh...I'm glad they concentrated on the single-player game. From what I've played so far, they've made an entertaining product.
What I would like to see, however, is an FPS where the main attraction is its online multiplayer. But either way, it's going to be hard for any DS game to keep players interested with its online component. Why? Because of people cheating online. I know I spend very little time with it; there just seems to be little point in it. I get no satisfaction from pwning without skill, and if the playing field isn't level, I lose interest. I think it's likely the same for most folks.
Which brings us full circle round. The DS is great for gaming on the go, and it can do things consoles cannot. The touch screen is a wonderful interface that offers a different experience for a huge variety of games, but it hasn't proven successful when it comes to online gaming. Even MKDS finally fell by the wayside. These days, players know that within the first week online gameplay will be tainted by cheat codes. I'm not making a value judgment, either -- just re-stating the obvious.
The only type of online FPS for DS I could possibly see holding any long-lasting interest for players would be some type of co-op.