I have a couple of electrical questions. First has to do with smoke detectors. I will be installing a smoke detector in each bedroom of my house and one in the attic and cellar. Do all of the smoke detectors need to be on the same breaker and wired together (interconnected)? If so I should be using 14-3 or 14-2 (ground wire used to interconnect)?

The second relates to Arc Fault Circuit Interrupters. I realize all outlets within a bedroom must be wired to an AFCI. What about the Smoke Detectors? I was thinking its better to be safe and simply put them on an AFCI. Will I be able to connect a 14-3 wire to an AFCI?

Use #14-3 with ground to interconnect all the smokes, do not use a ground wire for this purpose. Use a #14-2 to feed the first smoke then #14-3 to all other smokes.

A detector is not required in the attic, if this is just a storage space. If the attic can be lived in then yes one is needed. If this attic is just for storage at the most, dust and humidity will cause nuisance tripping. Detectors are only required on level that can be occupied. You may install one in the attic if desired, just be prepared to clean or replace it at some time, and experience false alarms.

According to the NFPA National Electric Code, a smoke detector wiring in a bedroom area must be AFCI protected. Some states, I know for a fact, have modified this code, and do not allow smoke detectors to be on AFCI protection, due to the conflict of interest.

If an arc is starting a fire, and an AFCI trips, then the occupant loses the benefit of the detector. You must check with your local and/or state code for their decision on this matter.

No, a three wire circuit cannot be connected to an AFCI breaker, these breakers cannot protect two circuits with a shared neutral.

Do smoke detectors need to be in series?

If you mean one after the other, no. Since the actual wiring is a parallel circuit, they can be wired in any fashion, as long as the 3 wire cable is connected at each location, all blacks together, all whites, and all reds, respectively.

They actually came up with "Wireless Interconnected Smoke Detectors". You could have 3 smokes interconnected on first floor, 3 wireless interconnected on second floor, If wiring is difficult to get between the 2 sets, use 2 wireless interconnected smokes, 1 on each floor. Expensive, but a time and money saver.
If interconnecting between detectors is a problem, then you just need to get power and use wireless interconnected smokes and it will solve problem and meet code.
They are not in series like a Fire System, where removing 1 would violate EOL.