It's WAY too early to start working on this, but I want to get this task identified. We need to create an upgrade path for any affected modules, CCK plus any core fields that get into D7 (profile? poll? taxonomy? whatever gets in). I realize CCK is contrib but the last time we moved part of CCK into core we found there were core implications for getting the upgrade working right and I assume the same will be true this time.
One thing that will help is that in every case the new storage location for the data will be different than the current storage location, so the data can live in both (or be retained in its old location as long as necessary). This is a little different than the D5 upgrade path where core used the same table name we were already using in CCK.
We'll need a matching issue in CCK for the CCK-specific part of this task, but I'm not going to create one until we get further along.