Justin Blanton explains how he writes on the Mac. It’s a great setup, and the same thing I use, except the TextMate bit. I prefer writing directly in Notational Velocity.
As he mentions it in his footnote, Notational Velocity can store it’s files (as plain text files) in any folder, including a folder in your Dropbox. I store mine there as a second-tier sync, knowing I can get to my stuff, even if Simplenote is down.
Update: Trey Heath chimes in on Notational Velocity as well.