Well let me put it this way, you have to change 1 part in your authenticator to make it Diablo 3 authenticator^^ ofc they will bring em out at some point, but it fully depends on what they got atm, if they got shitloads of normal battle.net authenticators they will try to sell those first, if they only got WoW and SC2 related authenticators, they have to bring Diablo 3 skinned authenticator out asap, cause who would really want WoW authenticator if they only play Diablo 3 and dont even own a WoW account
I think they will, no way to know for sure when though. My husband has a wotlk one, I have a cataclysm one, we didn't actually choose our designs though - just bought them at different times. There's an SC2 one so I see no reason they won't do a D3 one for people who only play Diablo. If they already made the design for Blizzcon then I'm guessing they'll offer them after the release of D3. Just my opinion.
It should be safe to buy one on ebay though if you don't want to wait. Probably pay more than waiting though.