⌘-I toggles italics on and off. Works almost everywhere. Highlight some text, hit ⌘-I, and the text becomes italicized– unless it was already italicized. In that case, the italicizing is removed.
Of course you can hit ⌘-I before typing something. If you do that, whatever you type after that is italicized, until you either hit ⌘-I again or click to type somewhere else.
This is very handy.