The summary of the latest White Paper is as follows:
In genealogy and family history, sources are critical for finding information and documenting where it came from. Because sources are so important, new.familysearch.org, Personal Ancestral File and similar programs, and Family Tree all let users enter sources. The Family Tree’s source feature is easy to use and will allow the tree to become the most accurate, collaborative genealogical database available.There seems to be some difference between entering sources and the programs "letting you" enter sources. I would certainly put Personal Ancestral File into the category of "letting you: enter sources. Letting you do it, as in the Personal Ancestral File program, does not mean that it does it easily or well. However, Family Tree is in a completely new category, entering sources is both easy and well done. You may quibble with the format, but all the essential information can be entered. If you are really concerned with the format of the citations, then you can format them in a word processing program and copy and paste them into the source fields on Family Tree.
This source White Paper specifically refers to a future feature of Family Tree for user uploaded documents and images.
It looks like Family Tree is about to become a very useful and interesting online program.