Omnidirectional short-medium range proximity sensing peer to peer network

Hi, I've been looking for a suitable and cost effective means of having several (tens) of arduinos send and receive location information within a smallish space (say the size of a warehouse). This would be for a sound based installation where people would walk around with an arduino based instrument that reacts to each other node on the network.

I'm aware of xbee and aware that it'd be way too expensive to use on this scale for two way communication. ultrasonic rangefinders might do the job but are directional-ideally this would be omni. GPS might work but might not be accurate enough given the short range.

I've read about cheap AM radio receiver/ transmitters which so far seem like the best option....anyone got any suggestions for where I might concentrate my research?

Cheers! -ed.

-ed.

hope it's ok to bump this in case someone knows of a way of doing ^.

GPS might work but might not be accurate enough given the short range.

And, of course, won't work indoors.

You are not the first person looking for this non-existent magic bullet.