I really like it, but part of me thinks the better way would be to just use whatever's on the title attribute of an element, which would of course need JavaScript. I guess this would be better for lighter sites.

Edit- I misunderstood how this works, I thought you actually had to add <span>s (extra markup) but it seems not! This is really great!

My only complaint is that it messes with elements if you're doing the :before/after thing for font icons.