The one article we (MDC moms the last time we discussed this) found was written by a chiro with no particularly specialized qualification in infant physical development or any other equivalant issue...and I felt it was very speculative. The front pack snugli has been popular since the 70's, even making the cover of the wall street journal - so I felt if chiros were going to see an explosion of hip issues they'd be apparant by now. She did a comparison with an eskimo tribe of some sort...theorizing an evironmental cause without knowing anything about their carrriers or lifestyle. So it was a weak article, based on speculation and not really any more definitive than what we as moms talk about on our own. I'd love to see something more well-researched - and I don't mean some super huge scientific study, either.
All in all, using one type of carrier or piece of equipment exclusively in my own opinion is probably not healthy...using a range of equipment and positions (even in a sling) is probably the safest since we don't have much helpful guidance out there.
edited to add, it seems like some of the criticism of front carriers is merely aimed at them because they are popular, used by upper middle class mainstream people, and don't look as natural/alternative as a sling. Cradle boards and carry baskets are found in many cultures too, but we're not promoting them as much. Anything that gets people carrying their babies close is a plus I think...whether sling, front back, or backpack. I am by far more worried about the amount of time children spend in car seats...which, if front packs are a concern, may well be far more serious for spinal development.