Good, well-thought out question regarding a blossoming trend in design.
Since we're not privy to Google's algorithm, and anecdotal reports are currently sparse on the subject, I'm going to avoid using definitive terms like "bad", and instead offer an SEO-perspective in terms of potential impact:
As you point out in Google's guidelines regarding hidden text and links, it's recommended to avoid using anything that might resemble "blackhat" techniques, which Google is targeting in its recent Penguin release.
<span> tags to add hidden text for icon fonts would seem indistinguishable from attempting to hide text for "blackhat" reasons, so it doesn't seem wise to attempt this. If you're concerned about SEO penalties, I would wait until reports confirm there isn't any, like through comparison testing, before using them.
The primary concern with icon fonts is accessibility for users with impairments and those using readers. There are ways to preclude icon fonts from being read out-loud, like applying aria-hidden="true" to your HTML element, or mapping them to characters as covered here, however, you should keep in mind that you're potentially losing out on the known weighted benefits of:
- Text in your content that can be indexed
- Text in
alt tags for images (as well as the
title attribute), which add internal links
In regards to the later, if you replace images with icon fonts, your page might appear as plain text only, potentially resulting in a slight downgrade.
To be on safe-side until the verdict is in, I'd suggest using icon fonts for decorative purposes, in addition to images, and avoid anything that's unknown which could result in a penalty, until demonstrated otherwise.