In case I want to use just RS-232 serial port, that would save me the RS-485 transceiver.
One thing to realize is that RS-232 and 485 are not protocols, they are specifications for voltage levels, speeds, wire lengths etc.
Whatever you decide on to cannot directly connect the serial port to other ports in such an environment, you do need transceivers of some kind.
RS-485 is generally recognized as being the most appropriate for several reasons including
a) Balanced line, not particularly upset by different GND potentials or noise.
b) Long runs, can go up to 1.2k, IIRC RS-232 is limited to 20m although I've seen it go a lot longer.
c) Transmitter disabling, with 485 you can disable the transmitter, this allows a multi-drop topology (essentially all nodes connected to one wire). RS-232 cannot do this and therefore can only be used for point-to-point.
So, in a nutshell RS-232 is not suitable, RS-485 is not only suitable but designed specially for just this sort of application.
I'm sorry, I don't get your point
No matter what transmission medium you choose you still have to design a protocol that involves either a master-slave arrangement where the master polls each slave in sequence (easy), or a multi-master setup that requires a lot of work to detect clashes etc (hard).
What my gadget is supposed to do is remove just about all that protocol work, you simply send data using the wire library and it is received by all Arduinos. It's a sort of "virtual I2C" link that implements a low-level protocol (the hard part), you still have to provide a simple layer above that.
Sorry if I've scared you off :), you can implement your own protocol fairly easily using RS-485 and a master-slave setup, however if you want a robust network then that's maybe just 30% of the job.
It depends largely on just how robust (reliable) you want the system to be.