What JonnieCache and DanBC say: keep the data encrypted at all times, then you can just overwrite the few bytes of the encryption key to render the data unreadable. This has the additional advantage that you only need to really securely erase a very small amount of storage (plus the controller's memory.)
(Editing my original post to make this more clear. Sorry.)
Oh, I thought you mean "as you push it" encryption.
When I was in the military, Encrypting data was not acceptable for disposal though. They make you physically destroy it, which is why I imagine they do things in this manner.
Yes, physically destroying drives prevents problems ("oh that wasn't encrypted?", "turns out it still had parts of unencrypted data on it from the previous server it was installed in", "what do you mean 'encrypted according to 1995 standards' (DES) is no longer secure?"). It's a good policy; but if you offer both "destroy" and "erase", "my" crypto implementation of "erase" has a lot to recommend it. (And there's no reason you can't follow it with a good zero-everything.)
That approach does have the advantage of speed, but it's not really a substitute for physically overwriting all the data.
History is rife with examples of people getting themselves into serious hot water by assuming that the lack of a well-known weakness in a cipher means that the cipher is secure. Anyone who's got an extremely high need for data security is hopefully keenly aware of that.
