Yes, it will be safe to pay for fast food with your smartphone. Smartphones have all kinds of capabilities now. You can do your banking on your smartphone, or use it to show your plane ticket when checking in for a flight. If you can safely bank with it, I'm sure it's safe to buy fast food as well.
Did you know Europe doesn't have credit cards like our cards? Know why? Because the technology we are using for credit cards is over 40 years old. In other countries they use smart cards with tiny chips to prevent or reduce stealing. The phone would be smarter and cheaper. No need to upgrade the equipment. No need to worry if you lose it. Every thing is backed up. Just go get a new phone.
If the Target breech taught us anything it was that no store or form of payment is safe from hackers. I don't believe the risk is greater with cell phones then it is with plastic cards. All of the information is electronically transferred no matter which way you pay and thus makes it susceptible to interception.
I use my smartphone on a daily basis. I probably depend on it more than I should. I do not think it is wise to use a cell phone to pay for fast food because if they accept it as a form of payment, every one will. I think loading payers onto a phone is dangerous given the NSA, government spying, and hackers.
Using your smartphone to pay for fast food purchases, will be about as safe as using your smartphone to pay for anything. Of course, we will have the great illusion of safety when it comes to paying via smartphones, but so did the millions of people who got their credit card information stolen whilst shopping at Target last December.