Why is the 3X+1 problem hard, by Ethan Akin, via Anarchaia.
You might also be interested in my previous post on the 3x+1 problem.
And welcome slashdotters! Will there be more or less of you than people coming in from reddit for my post on "Why g = π2"?? (I don't know yet. I don't know enough about the shape of a "typical" slashdot traffic spurt to guess.)