Blizzard game logo and name legalities.

I was just wondering if anyone had any insight on what sort of stance Blizzard takes on situations where a fan was interested in making t-shirts or other paraphernalia with names or logos that reference the games.

Do they attack with a vengeance or are they generally fairly lax with things as long as people don't try to pass of their stuff as "official"?

I ask because I was bored and made a couple Diablo 3 t-shirt logo prints just for kicks and they turned out kind of decent.