The rumour looked pretty solid, but confirmation is always swell. Konami have updated the original "next Metal Gear is..." site to confirm that, yes, the next Metal Gear game will be for the iPhone.
What's more, we now have some details on the game. It's an original title set in the MGS4 universe, and consists of 8 stages (with the promise of more to be made available later). It's obviously controlled via the touchpad (as opposed to the accelerometer), with a sniping screenshot looking a lot like the iPhone's zoom-in text editing setup.
It's due for a worldwide release in Spring (or Autumn - AU Ed) 2009.