Icon fonts is a cool idea, but they are not "> images".
They don't save additional requests (if you use spritesheets), are not smaller (2 color gif vs. full vector font), and most importantly, and why no decent designer would ever use them in small sizes, is that they absolutely lack any sharpness! Look at the QR code icon on their demo. It's a mess.
On the other hand, image icons can't change color on the fly or be dynamically upscaled, both of which are requirements when you are building a framework that is meant to be themed by users.
You can use png masks to create an icon, and then change the background color of the element below it. You can also use image scaling to make the icon any size you need.
Have you viewed them on a new iPad? Much sharper than PNG/GIF. Even on "standard" res tablets, if an image-based UI element is scaled in any capacity, the smoothing renders it as blurry as an antialiased font or SVG.
They don't save additional requests (if you use spritesheets), are not smaller (2 color gif vs. full vector font), and most importantly, and why no decent designer would ever use them in small sizes, is that they absolutely lack any sharpness! Look at the QR code icon on their demo. It's a mess.