Love the Maddie ones - here's some more:

What's the difference between Maddie and Houdini? Nothing - they both disappeared and they're both dead.

What's the difference between Jade Goody and a scooter? A scooter can reach 30.

Two nuns in a bath. One says "Where's the soap" to which the other replies "Yeah it does doesn't it"

A man takes a bath with his young son and his son points at his father's willy and says "Daddy why isn't mine like that?"
He replies "Well for a start son, yours isn't erect"

Also, regarding the clean joke, I heard a different version:

Do you want to hear a clean joke? A man took a bath with bubbles
Do you want to her a dirty joke? Bubbles is the 6 year old boy from across the street