Majenko..., I always thought RS485 took 3 wires, two for "Differential Data and one as ground... All three required... RS232 only requires two Tx OR Rx and ground.... Maybe you know something I don't?.
Three wires between the transceivers, but only 2 wires between the transceiver and the Arduino (at that point it's normal TTL RS-232).
But hey, there's no reason why you can't have multiple stations listening on a wire at the same time, as long as only one actively drives the line at a time. You could do it easily with no driver hardware whatsoever - just a bunch of Arduinos. When an Arduino's IO is in input mode it's high impedence - not quite like open collector, but close enough. Switch to driving the line for one station only at a time, and while open collector is not strictly needed (only one station driving a HIGH or LOW), it's a cleaner implementation. Alas the Atmel chips don't do open collector (or open drain as it should be - mosfets y'know - I hate the Atmel culture of misnaming things).
Yes, there are dedicated chips around for this sort of thing, and obviously using a higher voltage transmission system, like TIA-232, will result in longer distances and better noise immunity, but for short runs at low speeds that's fairly irrelevant (remember, TIA-232 can work at up to 300m or so with low capacitance cables).
RS-485 (EIA-485/TIA-485) is primarily designed to use a balanced line communication system, yes. However, there is no reason why you shouldn't take the principles of the operation of it and remove the balanced line portion of it. It just ends up as a simple serial stream with open drain transceivers on it which spend most of their time in listen mode.
As you well know, you can make any IO line an open-drain output by just slapping an N-channel MOSFET onto it, but making it bidirectional so it can read as well is more tricky. I find the easiest way is just to use a second IO line as an input, so you have send and receive always available - read from one port and write to another (just remember to write a 0 to the transmit line after you have finished). Yes, again there are dedicated chips, bi-directional open drain buffers, but for a single communications line there's little point in dedicating an entire chip with say 8 buffers in it, wasting 7 of them.