It's my understanding that the BA 787's will pass through CWL for initial training and then maintenance in due course.
Unlike the B773 the B787 will be delivered to British Airways with the cabin interior already installed.
One would assume that BA dreamliner will have their major maintenance at Cardiff as they will be replacing the B762 and some B744 however I have been reliably informed that no decision has been made as of yet.
As for flight training could be a possibility but i'm sure the first airframe is due this month have not heard any rumours of training as of yet.