I really want to say keep the MacBook Pros in the same article. I'm always thinking about this because of cells, but cells don't get nearly as much coverage, so often, it's less of an issue.
But the thing that does it for me in this case is that it will be unclear which pictures are the new and old gens. This is especially important with the latest revision because of the larger trackpad. I can see that being a deciding factor for some people (cause it's so damn awesome), so we need to be as clear as possible.
I agree with DUIU about the MacBook.
PS. To those of you who haven't gotten to use the MacBook Air's trackpad yet; it's amazing.