Mark Kermode recently wrote an interesting article about 3D in movies (http://www.guardian.co.uk/commentisfree/2010/apr/11/3d-avatar-hollywood) in which he points out the difference between a movie filmed in 3D (i.e. Avatar) and one modified afterwards (i.e. Alice In Underpants, Clash of the Shitans).
Personally, regardless of whether it's a "con", it makes my eyes literally water. If this is to stop happening then it will require a physiological change, which I'm not over the moon about. Plus, unlike an innovation like HD, filming in 3D requires special equipment which is not easily available to small film-makers, so it does not encourage independent productions. Finally, it's a freaking joke that I should go to my local Cineworld, present them with my £13 p/month Unlimited card and my own 3D glasses, and be told I have to pay extra, even though the 3D image is being produced on standard cinema equipment.