Instagram's API TOS has always prohibited third party clients. From the "You Shall Not" section:
"1. Use the Instagram APIs for any application that replicates or attempts to replace the essential user experience of Instagram.com or the Instagram apps."
Well the biggest difference is instagram won't let you post from anything that's not the official client. They kinda "baked in" the need for their app in the flow. With Twitter, it was possible to totally have a "custom" experience.
It's not too hard :) As you know official API doesn't support sending medias. You can send unofficially by using instagram websites URLs but you need users passwords. Its not appropriate to do in terms of user security.
Oh, I just want to write a short script to take my latest flickr uploads and send them to Instagram. I bet someone has already published some Python code, I'll look, thank you!
Instagram has filters in place that will flag / disable your account if you try to upload with anything but the official Instagram app. (that is, if they can detect that you tried that). You may have better luck using the Instagram hooks in a scripted Android Emulator
Ah, bleh, that's not worth it for the time it saves to upload a photo a week. What's up with all these closed apps that I can't even talk to the way I want to?
"1. Use the Instagram APIs for any application that replicates or attempts to replace the essential user experience of Instagram.com or the Instagram apps."
http://instagram.com/about/legal/terms/api/