http://www.webdesignfromscratch.com/...ity/#alignment
When reading text on my ebook reader, I get less on a line than in your average paperback (I keep the font size a bit larger) and much prefer left-justified text as it's easier to read.
For me, fully justified text belongs in multi-column formats or graphic-designed based items such as brochures, text books, etc where format often has significant meaning (isolating text groups based on meaning/value/etc) or significant graphic impact (look, a bunch of rectangles on a page).
In your average novel, the right-justification serves no purpose beyond aesthetics when you're looking at the image of the page, not reading the words.
In fact, my understanding from past education, research into it, and a quick check online to confirm my memory, is that reading fully justified blocks of text is a slower process than left justified, as the eye isn't able to, as readily, move from line to line in the text - it doesn't have a reference point for which line is which as it does when there is varying amounts of white-space to the right of each line.
For ebook readers, until they do a good job of on-the-fly hyphenation of the languages involved, I'd sooner have them leave the right-justification part of the equation alone. If I'm the structure of the document is that important, I'm likely using a pdf anyway and zooming in, and the point is moot.