twit.tv has an episode entited "The Kindle Paperweight?":
http://twit.tv/show/tech-news-today/577
It is the FAQ page linked in
jswinden's quote that still says the Paper White model is SO-only. The purchase page shows a non-SO option for an extra $20USD, as I mentioned previously.
If they do use the same firmware as a K5, it will need to support a different resolution and no Home button in addition to the K5 hardware. Or they may get goofy and have different flavors of the same major version numbers, for different hardware. Or they could go with higher version numbers. We will know when somebody gets a Paper White kindle and reports what version of firmware it is using.
There was version number confusion when the K4 and K5 were released, leading to messed up names like K4NT and K4Touch in speculative posts between product announcement and physical possession by members.