I'll look into recv; i hadn't thought of that!
The reason I don't have the sensors 'free-running' is that they're sonars, and they tend to interfere with each other unless you make sure they all take a measurement at the same time, or they make a measurement sequentially.
I'm attempting a 'polling' solution where one computer coordinates all of the arduinos to tell them when it's their 'turn' to read, and then the arduinos take a measurement and send the data along to their respective computers.