FWIW, the state of Oregon's chief inspector has issued an interpretation that says that the inside unit does not require it's own disconnect if the external unit which supplies power to the internal unit has a LOTO disconnect:
Indoor components with a unit switch that complies with NEC Article 424.19(C) or 422.34 are adequate to meet the intent of the OESC for personnel protection. If there is no switch on the indoor unit, Article 422.31(A) applies, and a lockable disconnect at the outdoor unit shall be permitted to serve as the disconnecting means.
The 18K BTU unit from Fujitsu I was studying has a plate for the inside unit at 0.3A (under 300VA), and a 64W motor (which at 746W/HP is 0.086HP), which is under 1/8th, so it falls under 422.31(A).
If you were to want to install a disconnect at the air handler though, you would indeed need to disconnect all three wires, including the communications port. The communication line, at least on the Fujitsu, is an optically isolated bidirectional serial communication link where either side of the wire can supply power, and either side can drive the wire from one pole to the other (changing the serial bit from high to low and vise versa). This is also how it appears multiple indoor units can communicate with the outdoor unit as the communication link is more of a bus and not a point to point communication wire.
The communication line is not standardized across brands, so other makes may vary, but I think it's safe to assume that with the condenser powered on, that the S3 communication line could carry line voltage.
The other question that comes up frequently is how the indoor unit can be wired using 14/4 if the primary is 12/2 breakered at 20A. At least on the Fujitsu, the secondary wiring to the indoor unit is fused at 5A inside the condenser. I would be surprised if others that required a larger breaker than 15A didn't have the same type of scheme.
Hope that helps.
