At first, I thought that was a poor choice, too. However, <i> has come to mean that something is to be interpreted under a different voice or mood. You are providing augmented meaning. To further color a bit of text with an icon, I think, is in the spirit of the rfc.

Notice, also, that the rfc talks specifically about classifying the alternate meaning, appropriately, with the class attribute.

http://dev.w3.org/html5/spec/Overview.html#the-i-element