iPhone CoreText: Find the pixel coordinates of a substring

Here's a screenshot of the twitter app for reference: http://screencast.com/t/YmFmYmI4M

What I want to do is place a floating pop-over on top of a substring in an NSAttributedString that could span multiple lines. NSAttributedString is a requirement for the project.

In the screenshot supplied, you can see that links are background-highlighted, so it leads me to believe that they're using CoreText and NSAttributedStrings. I also found something called CTRunRef ( http://developer.apple.com/library/ios/#documentation/Carbon/Reference/CTRunRef/Reference/reference.html ) which looks promising, but I'm having trouble fitting it all together conceptually.

In short, if I have a paragraph in core text and when I tap on a word, how do I find the bounding box for that word?