As of now, unity itself isn't themeable - the theme doesn't change - only the window decoration changes, if that makes sense. If unity is to become default in 11.04, surely it would make sense for the launcher and task bar to change as well?
Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.
No decision has been made on this yet.
Keep in mind that we will need something to handle having a high and low contrast theme for accessibility reasons; and that is very much a high priority, so we'll have to revisit this after conclusions have been made at the Ubuntu Developer Summit.