yup - down here in Oz, "creasing yourself" is a very common phrase - and it generally means being very amused indeed - I like Bingley's allusion to laughing so much that you bend double.
Also - "taking the mickey" is probably even more common here although I have no idea of it's origin. I am fascinated by the the "micturation" suggestion, but can't relate bladder contractions to sarcasm. Well, not easily anyway. . . .
An equivalent (albeit less common) slang phrase to "taking the mick" is "having a lend of", as in "Do you really like my Hawaiian shirt or are you having a lend of me?"