MarkdownMail lets you send email that has been formatted using John Gruber's Markdown syntax. The newest version has been updated to be a universal app for iPad or iPhone/iPod touch devices. It's US$4 for new users, or a free upgrade for existing users. The new version also fixes some bugs and updates to the latest TextExpander touch SDK.
The iPhone version looked good, but I knew that I wouldn't want to use it scaled up on my iPad. As soon as I saw it was available for iPad, I grabbed it. The price has increased from $2 to $4, but hey, sometimes being an early adopter pays off. Those who jumped right in showed the developer there was interest in the app, and that interest helped support the development of the iPad version.
I like the fact that it will preserve the Markdown formatting when sending (rather than converting it to HTML before sending). It also has a nice preview button to show how the text will look when it's converted. Despite the name, you don't have to email the file when done. There is an option to have MarkdownMail convert your text to HTML and copy it to the clipboard right from the app.
I'm still putting it through the paces, but let me know if you have any questions. So far, it looks very handy for folks who like to write Markdown on the go.