Is "bless your heart" something only used by old women in the South (all I've ever heard)? Or is it ever appropriate for a man to use it without seeming unmanly? Does the term always have condescending connotations?
I don't recall hearing it outside the South (although on Mark Twain's evidence it seems to have been very common once in Missouri), and I've been back to the South very little in the last 20 years; but when I was growing up it was used by men and women alike, though more by women than by men.
It can be said with a forgiving intonation (“Bless your heart, how could you be expected to know?”), which inevitably has something of pity and condescension to it, and it may be ironic; but I cannot remember hearing it used sarcastically.
Its primary use, in my youth, was as an expression of admiration or gratitude for a generous action, where a mere “Well done” or “Thank you” was felt to be insufficient. It was spoken then as three stressed, spaced syllables, with a drop of at least a major third to the last:
Well, b l e s s • y o u r • h e a r t !
Well, Mark Twain certainly used it. See his essay Fenimore Cooper's Literary Offenses, in which you'll find this passage:
We must be a little wary when Brander Matthews tells us that Cooper's books "reveal an extraordinary fulness of invention." As a rule, I am quite willing to accept Brander Matthews's literary judgments and applaud his lucid and graceful phrasing of them; but that particular statement needs to be taken with a few tons of salt. Bless your heart, Cooper hadn't any more invention than a horse; and I don't mean a high-class horse, either; I mean a clothes-horse.
It's an older expression, to be sure, and if you hear it today it's probably from a genteel old lady or gentleman — or being used sarcastically by someone else.