I presume you mean running MacPorts in interactive mode. If so, a substantial change between 1.5.2 and 1.6.0 is that readline support was switched to off by default due to the sheer amount of compile time problems it caused, and not having such support would definitely explain the behavior you're seeing. Selfupdating preserves some settings across upgrades, but I don't think it has the ability to detect whether the installed MacPorts had readline support integrated and preserve it as appropriate. I'm assigning this ticket to James (interactive mode author) so that he can comment on the matter.
In any case, one quick way to get readline support back in is to install MacPorts from source. Download the tarball and configure the sources with the --enable-readline
switch to the configure
script.
-jmpp