In the /badges page for a logged-in user, there is a green check beside each badge that the user has obtained. This check is the unicode symbol ✔, but for many users that will display as a block. This is the case on a default English Windows XP install (I think because Arial Unicode isn't there by default), which I imagine covers a huge part of your audience.
It would be better if you just used an image.
Here is a screenshot: