Found this article; thought of Alissa.
I don't know that I have much to add to the conversation, besides agreement that, yes, it's creepy to look at a display of books and see only torsos, or feet. Lately, when we do get a face, there's a good chance that it's the same face on a different book. Book covers are supposed to be dynamic marketing tools, pulling potential readers in with an interesting image that will set that book apart from all the others.
(The reused-images things isn't limited to teen books, nor to covers of faces. But you knew that.)
So how do you get a teen interested in a book that looks just like all the other books? How do you convince someone that Sarah Dessen's books aren't as fluffy as the covers suggest? Or that House Of Stairs is still a creepy read and holds up well, even though the only in-print version hasn't had a cover re-design since 1991? It's easy when it's a teen who trusts your judgement--one you've recommended books to before--but when it's a teen who hasn't been in before, or hasn't spoken to you before, it can be hard to overcome the decisions of the publisher's art department.
And this isn't even getting into proper representation--the great Whitewashing debates, or the unwillingness to show overweight or unattractive girls, even on books about ugly and/or fat girls. That's a whole other issue that's been covered elsewhere.
So maybe this is more of a link round-up than any commentary on trends, or on the original article. But the fact that I can do such a link round-up--without much effort, even--says we have a number of problems that need solving.