I'm not a huge fan of the strikethrough, because it has been abused in the past (people crossing out insults as if it is any less insulting). I can see the advantage of it in sales posts though, and hope that it is mostly restrained to that area of the board.

Anyway, I've finally been able to track down the info and icon, and it can now be done with any post or PM. It is the one at the far right on the row of coding buttons when you type your message/post.
Use it like the 'quote' button, to wrap around highlighted text, or to activate it and type in between the 2 tags like this.