You didn't mention what sort of stuff you would be writing. But for simple note taking I like PlainText:
http://www.hogbaysoftware.com/products/plaintext
It syncs with Dropbox and creates plain text files that can be edited in TextMate.
Mainly note taking, although the files have gotten rather large .. need to toss the cruft!
The high order bit is that the editor work well with textmate. I think that means utf8 & LF and no hidden metadata. So I'll give PlainText a shot.
While looking at the iPad text editors, I saw Markdown mentioned. I hadn't really considered it before, but it looks like a nice way to structure text files with the least intrusion possible. I may give it a shot, and I noticed TextMate has a plugin.
-- Owen