I think that in general, the iPhone screen size is too small to accomodate the numerous buttons and HUD in most FPS games. I found it frustrating to try and see the game with my fingers obscuring a third of the screen. The gameplay videos for NOVA and BIA look great on the iPad, although I haven't tried them yet.
I've played Call of Duty: Zombies and Duke Nukem 3D on the iPad. And although I'm a huge fan of FPS games both on consoles and PC.... I just could not get used to the touch screen controls. I wish I could, I'd love to play those games. But I felt super handi-capped with the controls. Perhaps I'm just terrible at picking up this control scheme. I felt like I was spending way too much time aiming. It was like my character was drunk.