[Info-vax] OT: ZIP/Postal code geo-coordinates

[John Smith (not the one @ HP)]

I need to be able to calculate/lookup distances from a specific postal/zip 
code - eg. within 50 miles of 02138 or 100 km of T0K 0Y0, or equivalent 
other nationa postal routing codes.

Just wondering if anyone can point me to a source of information that 
provides presumed center point latitude/longitude for a given zip/postal 
code in US/Canada/Western Europe as a starting point?

I've been poking around on various national postal service websites looking 
for something like this but have not stumbled upon this informtion.

Thanks.