@Mr. S&N: Of course with someone you know well, e.g. your spouse, you can have that covert channel without the technology, just subtle use of word choices, prosody, facial expressions, etc… Oh no! Is humanity losing that skill that we need to "text" our subtext? :)
@Theodore: Sometimes, though, you're doing so among other people who know you well and would be able to pick up your subtext. My spouse and I make use of text and instant message in this way when we're among family or close friends, as a way of keeping the subtext from view.