That being said I have to think a 0-confirm ATM is likely a bankrupt ATM.
Double spend = free money (literally).
Maybe it could be programmed to allow to have only X coins pending confirmation, it would greatly limit the incentive to attempt double-spends.Or maybe you could send funds to the ATM while not being right in front of it, in advance, once the transaction is confirmed you get a receipt that is redeemable for cash at the ATM.
The first one is a non-starter. If the limit is too low the ATM is useless to any legit customer and yes some crooks will still attempt to steal a token amount of coins (ask Gavin the amount of fraud the bitcoin faucet gets and that is 0.01 max payout). If the limit is too high all crooks will just steal that amount.
The second is a little more interesting and something I thought of yesterday when walking by a Rebox machine. "Normal" ATM are mainly about impulse decisions ("oh crap I need $50") so having a call ahead normal ATM is kinda dubious. A Bitcoin ATM is to provide planned fast exchanging of currency. Nobody has a problem with reserving a movie online w/ RedBox.
A BTC ATM could operate in a similar manner. You install an app on your smartphone (could also access it from any web browser). The app generates a one time deposit address and current exchange rate. You transfer the coins to that address via whatever client/wallet you like and the app provides you a "withdraw PIN" and block counter. To use the ATM you just punch in your one time withdraw code (or have your phone send it via NFC or QR code). Obviously other options are also available even optional magnetic cards/accounts for frequent users.
Now if you decided not to "pay ahead" you could at the ATM could replicate the same process except you will need to wait 6 confirmations. Maybe have a 3rd party provide "double spend insurance" and allow the customer to cut that down to 2 confirmations for a higher convenience fee.
Obviously cash-> BTC tx would always be instantaneous. Also as someone mentioned a smart ATM would adjust its exchange rates to reduce or eliminate the number of times it needs to be refilled by influencing customer behavior (maybe even allow users to sign up to learn when ATM has a good "deal").
The real problem w/ BTC ATM is density. Normal ATM work because within 10 miles there are thousands of users with compatible cards. I doubt there are many locations in the world where BTC account holder density is > 100 in 10 miles radius.