All the time - I used to clean a dirt ring off the bathtub with monotonous regularity but then I got smart and now if I see that they're extra dirty I hide the plug and make them shower. They look so funny before their shower with dirt all over them and I get a wierd kind of pleasure from seeing a real result from a shower - the sight of dirt sluicing away always cheers me up (yeah, I'm a freak).
We built a sandpit and they dig through the sand to the dirt underneath and now they only play in the dirt corner of the sand-pit.
What they really enjoy is when it rains and they can play in mud. I will admit that even at the ripe old age of 34 I enjoy squeezing mud in my toes, it's such a nice sensation.