Given a Salesforce pod/instance name such as NA3 or CS6 how can I find it's physical location?
You can do a DNS lookup to find what it is an alias for. E.g. for na5 via DNS lookup.
na5.salesforce.com is an alias for na5-chi.salesforce.com.
na5-chi.salesforce.com has address 184.108.40.206
na5-chi.salesforce.com has address 220.127.116.11
Note the na5-chi. That would be the CHI airport code - so somewhere in Chicago.
Again for cs6:
cs6.salesforce.com is an alias for cs6-tyo.salesforce.com.
cs6-tyo.salesforce.com has address 18.104.22.168
cs6-tyo.salesforce.com has address 22.214.171.124
So TYO - Tokyo.
Interestingly enough, the location based on the IP address alone is Singapore.