There are a couple of subtle yet very important details that are always overlooked when trying to compare the hardware of these two devices. First, the iPhone uses an ARM processor. This is the single worst production level implementation of a RISC style architecture that you can find. It was really meant for extremely low power devices. (typically cell-phones and old PDAs). It has gained quite a bit of popularity in recent years as it has been improving. It even somehow made its way through nvidia's doors. However, quite literally, every other risc architecture out there is better. The PSP uses MIPS. While this really isn't a whole lot better, it does make a difference.
Another important thing to remember is that the PSP has a 128 bit bus compared to the iPhone's 32 bit bus. I don't know how it is that this is always overlooked, but this makes a huge impact in overall number crunching (which is mostly what gaming is). This is the same advantage that the Neo-Geo had back in its day that made it appear so far ahead of its time.
The third point is that people tend to drastically under-estimate the role the CPU plays in 3d acceleration. There is this myth that the cpu doesn't matter as long as we have a great GPU. This happens to be somewhat true in the PC world because x86 processors have gotten to be so fast that they can generally keep up with the GPU without too much effort. In the mobile world however, this isn't the case at all. These dinky little ARM processors are fighting for every breath, and when they are responsible for keeping every other piece of hardware up and running, they're not going to have a lot of free cycles to give to the GPU. I think that this is the root cause for the iPhone's inability to game on par with other mobile devices like the PSP. No amount of software improvements are going to fix this. They either need to have some type of "game mode" where the entire phone minus the GPU, audio and screen are disabled, or move to a larger buss (which I've heard rumors that the next gen iPhone will be 64 bit). Even then, I'm not sure that it could keep up with the PSP.
One last point, remember that Carmak is pure software guy, a mighty fine one at that. However, I wouldn't put muck stock in what he has to say about hardware. Just my two cents.