alt131 - 10:32 pm on Jun 21, 2012 (gmt 0)

Well, you don't know that it will be a single line do you? Maybe your user is reading it on an antiquated phone with a display that's only ten characters wide

Two issues. The one you're referring to is the displayed width of the lines of content. The other is content that by convention only occupies one line. For example:
All the world's a stage,
And all the men and women merely players;
They have their exits and their entrances,
And one man in his time plays many parts,

Even the punctuation reinforces it was never meant to be written down as: All the world's a stage, And all the men and women merely players; They have their exits and their entrances, And one man in his time plays many parts,

Even more so with things like Haiku, where the number of words/syllables on each "line" and the shapes of the words and letters themselves creates a visual picture that is as important as the meaning of the words themselves.

A collection of lines is not a paragraph, but the individual single lines (or even single words) aren't a paragraph either. We just don't have an element to mark these things up correctly. Hence a variety of methods as suggested here.

That's before we get to the issue of what users (who like to surf at 10 characters wide) do to your beautifully crafted layout ;)

So what would be "perfect" for a one-liner?

Hence I say go back to the content and the context - is that really a "single line", or something else such as a quote, sample, maybe blockquote, perhaps it is really a sub-heading. Part of my thinking is that aside the challenging cases, there is something suspicious about have a "single line" in the middle of a "para". I know we see them all the time ... but is it "correct" or just poor grammar?