Eh..I can see it having some more in depth games on it then the mini games you can already buy off of the stores but it becoming an actual platform? Doubtful. It's possible but in the comparison to gaming platforms such as The Nintendo DS or the Sony PSP, I doubt a possible iPhone gaming platform would cause many problems.
On another note, the DS already has a touch screen. The iPhone is way too overpriced to be only bought for gaming, so for those who already have it and are planning to buy it for its main features will really be the only ones playing it.