Not completely true. It runs a cut-down version, much like Windows Mobile is a cut down XP. It would require huge resources that the iphone simply doesn't have to run OS X fully.
It's a nice interface, Apple always does the software aspect well. But the hardware lets it down. And on an unproven, new OS, there'll be no 3rd party apps, no online community to support it and develop software, and hence less customisability.