its going to work just like this. When your playing music you will have the controls on the sides, butbin land scape mode. It would be left arrow to move left, right arrow to move right, you would shake the iPod phone up to make your charecther go up since iPod has anaccerlaromintor in it(I spent that wrong) and it would have a letter "a" button and a "b" button also.
I'd would photoshop it if anybody would like.
Well, shaking would inhibit moving the crosshair up and down really quickly (some people actually do that to signal to their teammates "yes"...it's hilarious). And to shake would also be bad, since for that one whole second, you can't really see much since you're shaking your iPhone (yeah, I'm a pro-gamer...then again, I wouldn't play on the iPhone in tourneys...but it would bug me since I'm so used to it, lol.)
As for the left and right and a and b...sounds like the gba, nes, and psx emulators...heard they weren't that great.
I guess you could have on overlay d-pad (not just left and right, since you need to walk forward/back and your accelerometer is controling the crosshairs) I've always found that even if the screen is big, if the game is 4:3 and the screen is 16:9, it bothers me a ton...or if my fingers make it a 4:3.
Now, the A and B can control secondary (zoom for most games) and primary (tapping the screen like Apple's fighter might work too). Then you need more buttons for "use", crouch, switching guns, etc.
I dunno...I'll have a go at a simple interface. Sadly, I won't be able to see it anywhere other than the simulator (dang it Apple, approve my application so I can pay your freakin $99!)...who knows? maybe there are some APIs hidden that work well. I'll let you know if it does work

. (stupid Apple, let me have my identifier!)
(PS. I guess I can't make generalizations for all FPS now that I think about some other games...I play Counter-Strike professionally, and there you need the entire left half [or right if you're left hander] of the keyboard bound.)