A quick calculation says the GPU in the iPhone 5 (an SGX543MP3 at 266Mhz i believe) can churn out 34GFLOPS. A current high-end desktop video card can take that to >3000, so I guess you can say an iphone has 1-2% of the power a high-end PC has.
A current-gen, bargain basement bottom-of-the-line desktop/laptop GPU still has 100+GFLOPS, so you're looking at 3x faster even there.
This is only theoretical performance mind. In practice, platform differences make the comparison very hard. The iPhone is more like say the xbox or playstation for developers - we only have 1 (or at worst 2 or 3) set of hardware to deal with. That means we can optimise like crazy for that particular set of features, and squeeze a whole lot of performance out of it.
On a PC (or mac) you have a near infinite number of CPU, GPU and memory combinations to deal with, so you can never optimise for all of them - in fact often you're just optimising for the lowest platform you can, and throwing some extra stuff in to please people with high-end machines. Most of the time you get nowhere near the hardware limits.
By the way, the Atom chips from intel actually contain a GPU that's pretty similar to the iphone's. They use the same SGX series, but different parts, and at much higher speeds. I don't think they use multiple GPU cores though, so it's entirely possible that an iphone 5 considerably outperforms a netbook for graphics!