How elaborately do you intend to format the thing? If it's just a bunch-'o-text, you could dump it into any text editor, do whatever minor cleanup you want, then run it through MobiPocket Creator and be done with it.
Keep in mind if you're creating flowable text, WYSIWYG no longer has any real meaning. The user can change font size on all Kindles, and screen orientation on the newer ones. What-they-see-is-not-what-you-made.