Many people have questions about vertical positioning of font as icon in a inline context. This happen because the icon are never perfectly centralized.
Put display inline-block and vertical align middle on icon maybe be the best solution.
One important thing is, as viewed in http://fernandoperigolo.github.io/font-as-icon/, when you increase the icon size, the browser try to centralize. If you have a even icon height and a odd font size, browser try to do the better, but not always we get a pixel perfection centralization.
To make this example, i use http://fontastic.me/ to manage the icons. It is a realy nice tool.