you need to completely reinstall macports 1.8 when you migrate from 10.5 to 10.6. macports does not support migration. the tool(s) won't tell you that, but will give various esoteric messages that will frustrate you because it ALMOST works if you do various forms of manual upgrades. but you'll be chasing those problems for a while. these problems were similar when historically migrating from tiger to leopard.
one of the key things Apple is trying to do with Snow Leopard is to surreptitiously get the whole world to migrate to true 64-bit computing, and to abandon PPC as well and fat binaries and all that jazz. those are the two underlying shifts in environment that will cause problems for you if you try to do this migration piece-meal.
after you've uninstalled, you also need to install the new xcode version. it'll be on the Snow Leopard DVD, but it won't install unless you explicitly install it from the optional folder.
before you uninstall (or just move /opt/local), make sure to save the output of "port installed | grep active" to a text file so you can reference what you had installed. then do a clean new install of macports in the usual way, and install the packages from your list that you know you'll be using again.