fantaxy,
I'm sorry I can't help you. I even don't understand what is your problem.
I tested your text on my Kindle 3.1 and saw spaces appropriately. See the attached screen shots. The first file, 6739 on the left, is of a raw UTF-8 text file, and the second, 6740 on the right, is of a .mobi file created with kindlegen. (The .mobi file is also attched.)
Or, am I missing something?
-- Alissa