I have a big project (interactive theatre) where eventually I would like around 100 Arduinos networked. All nodes need to tx/rx with all other nodes, preferably in a multi-drop/daisy chain setup, but a star may be ok too.
RS485 is nice, and I've seen it pushed over 100 devices with repeaters.
Another option, is to daisy chain using two RS232 ports per arduino and creating a repeater out of each arduino. It's more complicated because you only have one HW uart, but it should be doable. On dual uart systems I've worked on 80+ node systems made this way. Latency is kind of high and band width kind of low though.