Ah HA!

Old wives say: Don't crack your knuckles -- it causes arthritis.
"The 'cracking' is actually caused by the bursting of a bubble of nitrogen that forms inside the joint when the joint moves," says John Klippel, M.D., medical director of the Arthritis Foundation. "Popping your knuckles may be annoying to others, but it doesn't cause arthritis, enlarged joints or musculoskeletal problems."
Final verdict: False

To me, this is really good news considering I crack my knuckles frequently. I think that may come from too much martial arts. ;-)