There have been several recent questions migrated to SU where the users asking the question and authoring existing answers are not SU members. Example: Windows 7 Intranet Web Service Name Resolution.
The user names are greyed out, indicating that there is no way to communicate with them. It's great that questions and any existing answers are migrated to the right site, but how can they be answered here if there is no way to get clarifying information or feedback on answers? How can the OP accept an answer?
Or is SU membership automatically transferred and that process just takes a while to catch up?