I'm assuming that squid keeps a list of internal ip:port that a request is made on and the matching external ip:port that the request is fulfilled with.
In the case of a long transfer, such as a file download, it would be nice to be able to see which internal ip:port is downloading the file.
I am able to see the traffic and get the external ip:port that squid is using easily with tcpdump or iptraf but I can't find a way to map this back to an internal ip:port.
Squid's access.log should show you the requesting IP address. I use SARG to parse access.log into something more readable. In the case that you want to know the requesting IP that goes with a specific file
...should tell you the requestor's/requestors' IP.