To me this comment does not describe the situation correctly (the document says, how it should be, not how it is). I use normally 'sudo port selfupdate' or 'sudo port sync', the result is that the subversion working copy owned by me will be tainted by files owned not by me, thereby preventing me updating. I am happy, if I really am wrong!
PS This is easy to fix (use chown before svn up), an annoyance nevertheless!