Easily de-anodized and polished, but not restored unless you've access to an anodizer.
I've polished the Cinelli sleeves on their handlebars, and there's a fine line between losing the logo or having it show through a highly polished sleeve.
I'd imagine it's quite the same for some Cinelli stems. I have a stem with what looks like a silk-screened logo. I've never figured out how to polish up that one without losing the logo.