I'm blogging this in English in hopes of providing a solution for other people suffering of the same problem.
I just spent a few hours investigating why installing MacPorts on Mac OS X 10.5.2 (Leopard) was failing with a 'postflight failed' error. Strangely even 'telnet rsync.macports.org 873' would cause a Connection refused error, though the same host worked when contacted from other computers.
Eventually I realized the problem was PeerGuardian, which was blocking the macports.org IP range. After unblocking it, the installation completed with no problems. Also 'sudo port selfupdate' works properly now.