Two of the biggest changes on the web recently are custom fonts (thanks to font-face) and responsive design.
One of the problems with responsive design are resizable images and by extension icons. When the interface adapts and changes size this can make displaying crisply rendered icons difficult.
Fortunately a few smart people have turned to font-face for the answer. By turning icons into fonts they become styleable with CSS, are infinitely scalable and are a smaller file size than imagery.
If you are interested in adopting this technique there are two options available to you.
My preference is to use a royalty free icon set that I can host myself. This will perform better than relying on a third party service.
Either way, I suspect font-face is the future of icons.