If I traveled more for work or if I was a bus/subway/train commuter, I would play mobile games. However I don't so my XBOX 360 is my gaming platform.
I think iPhone and iPod Touch will do well for games. They have some good and impressive games already but I don't believe it will push PSP or DS out of the market as some suggest.
The iPhone / Touch game controller interface, or lack of such, makes some games ridiculous and un-playable because the touch screen and accelerometer are not good control mechanisms for many games.
Like the Wii, I believe good games that suit the touch/motion control mechanism will do well and help it be successful and I think it will find a successful niche alongside PSP & DS.
But in reality, they have a whole market of iPhone customers, and the whole market of iPod touch customers..lucky for Apple!