Following the advice found in many places, I tried to install Ruby 1.9 without a suffix using this command:
sudo port install ruby19 +nosuffix
This still leaves the suffix "1.9" on the binary; typing
which ruby yields
/usr/bin/ruby (the system installation); in Macports' /opt/local/bin, there is only the file ruby1.9 to be found.
This is on a system with a fresh installation of Macports. No error messages or warnings during the execution of the above command.