Pullum keeps a slightly updated version of this poem on his website: http://www.lel.ed.ac.uk/~gpullum/loopsnoop.html

It adjusts the phrasing in a few places to make the meter flow more naturally. Apparently it also fixes a small error in the original proof, which, for the life of me, I cannot find.

I've used this poem to teach the halting problem to a variety of people for almost a decade and a half. Reading the one linked here just didn't sound right.