Iceman...
In the US, zipcodes (mail) are 5 numbers, area codes (phone) are 3.
Honestly, I think it depends on the area code. I think a domain like 212.us (New York area code) would be way more valuable than say 907 (an area code in Alaska). On the same token, they're all 3-number .us domains. Unless the buyer has a specific use for the particular area code, they'd probably be just as happy buying another 3-number domain. But, If the area code has a substantial population living there, there is naturally more potential for buyers and the posibility of finding someone who will pay a higher price. On the same token, popularity can rank high. Think of the zipcode 90210.
Just my two cents, plus a rusty nail.
-Nitro