Google around for "custom holographic security stickers". The people who make those will work with you on the design and whatnot.
And yes, they are stickers on Casascius' coins. If you want to mint the 'holographic' effect into the metal, you're looking at a fairly expensive set-up fee.
If you do happen to make them - good! Seeing as Casascius isn't really mass-producing them, and a lot of people love the idea of physical bitcoins, even though kind of defeat one of the points of bitcoin