In every previous version of Ubuntu, I'd just right click the file, hit Properties, then one of the tabs would let me add all the custom icon tags I'd like. It seems to be completely missing in Oneric, though I can still see the icons on files I've tagged previously, there's just no way to change them.
The Gnome 3 devs have removed the Backgrounds and Emblems tab for the following reasons:
There are some reasons for it:
the code is quite old and crufty, pulls in a lot of libeel code and is basically unmaintained.
the feature, as it stands, is not really useful, and it's more cosmetic than anything else, especially now that we use browser mode by default.
we're trying to achieve a more streamlined nautilus and removing legacy features goes in the right direction.
The article goes onto mention that the underlying API is still there and certain apps like DropBox can & does make use of it.