Well, the launch show for BB11 got 5.8 million, so 5.1 million is pretty good when you consider that they'd expect lower figures anyway because of the station change (C5 generally does poorer than C4) and it's the celebrity version (which generally does worse than the "proper" BB).
The real question is how many viewers stick with the show. My guess is that the figures will drop dramatically very quickly. Remember that some of the "casual" viewers last night will have been expecting the likes of Charlie Sheen and Pamela Anderson, and there has also been a huge amount of hostility on F*c*book regarding the lack of Live Feed.
The Star will do its bit in drumming up interest, of course, but continually putting Amy Childs on the front page can only do so much. There have been suggestions that the success of the launch show will encourage other, non-Desmond tabloids to cover the show, but I doubt it. If CBB doesn't get an online "vibe" going either: well, we'll see...