There might be some other way, but for now what I had to do was to save the HTML generated from the Markdown conversion to a temporary file, then open that in Safari, have Safari select all and copy to the clipboard (copying an HTML object), then having Mail paste it – replacing the selected Markdown text. So, the service workflow takes the selected text, but does not itself replace the selection directly.

A PHP script and style file for generating rendered HTML from Markdown for pasting into mail messages. I use it instead of WYSIWYG editing in Rich Text mode because I'm so used to working in Markdown.

I run this from TextMate using the Mail Preview command:

Looking at your three examples, I would say the following:

The first one (hyphens) emphasises that it's his third meal of the day, making it at least as important an idea as the broken table.

The second one (commas) downplays the idea, making the broken table more important, just leaving us with an impression about the eating. This is the most subtle of the three.

In the third one (parentheses), the narrator is speaking directly to the reader and is making an out-of-character remark about Bilbo's eating habits. This is the most blatant of the three.

These features include the ability to use the “esc” key to close the compose window, type anywhere to tweet, user timeline derepeater, as well as others. Here’s a screenshot of the secret preferences pane in Twitter for Mac, which you can activate any of those settings in terminal. I will update this post as I find the other terminal tweaks.