Let's see... FB launched the mobile "share" feature this December. Twitter launched retweet in 2009. AFAIK tumblr has always allowed reblogging. Finally, Soundcloud just enabled re-posting this December--it used to be a primarily "create and upload" community but now you can repost someone else's stuff.

Don't get it twisted. I like Instagram the way it is, and it may never allow it. That said, I think eventually they'll bite the bullet. My guess is late 2013 to mid-2014. What do you think?

I think that pre-FB, it could have been something on the to-do list. But now, I think that Instagram exists more to push items to other places, especially Facebook, and build the "social graph". But, that being said, I wouldn't put it past the management to allow it. Can you re-blog, retweet, or share something that's "private"?