Recommendations for a clean, simple, functional writing app that will sync documents between a desktop Mac, ipad, and iphone/ipod touch. I've been looking at WriteRoom and Celtx, but was wondering if there are any apps that anybody loves for writing between on all three devices. The text is a book that I'm writing which will contain italics and bold, so it can't be completely barebones.

Because it's not a personal or classified document, the use of dropbox is okay.

I don't want something overly complex or cluttered.

Any recommendations? What apps are great for writing?

Thanks!