[lobster_johnson]

That's not really a solution. That will create an awkwardly ragged right margin. Justified text is more readable and makes text look less messy.

[mattcurtis]

I like the Knuth/Plass technique as well, it creates much more uniform and visually appeasing line margins. The reason something like it would be so difficult to implement is that it requires heavy geometric processing every time the content reflows, which would impact Hyphen's performance a lot. Another issue I see is that it would pollute an ePub's DOM more than I'd care for.