Either you upgraded to Snow Leopard, or migrated to this Snow Leopard machine from a different machine, and did not perform the Migration procedure, or you subsequently installed a 3rd-party package that itself was built using MacPorts which overwrote MacPorts-installed files like your zlib library. In any case, the solution to this specific issue is to rebuild your zlib:
sudo port -n upgrade --force zlib
However, in either of the above scenarios, it's likely other libraries are affected as well, so you should probably uninstall and reinstall MacPorts and all ports.
