Jon Alter runs the math for what Hillary has to do to get back take the lead among pledged delegates.

Late Update: A surprising large number of readers have written in to note that Hillary Clinton cannot "get back the lead" among pledged delegates because she has never been in the lead. To the best of my knowledge this is true. Obama has been in and maintained his lead throughout. I'm not sure this makes a great deal of difference to the larger story. But accuracy counts. So I stand corrected.