My 2.5 year old developed one on his back shortly after birth. Our dr said it should fade by time he was a year old. At 12 months, it was unchanged, but now, it has shrunk to less that half it's original size. I estimate it will be gone by time he is 4.
My second son has a much larger one on his forehead. We get the stares and comments all the time. I just tell people it's a birthmark and it doens't hurt him and it will go away on it's own.
His dr (a different one that my first son) used to be a dermatologist. She said she saw lots of these H, and that they mostly all disappear by time the child is 10. His is supposedly starting to involute, but to me, it just looks puffier, which I guess is probably part of the process. I do see the grayish spots that resemble dry skin on the H, which is a sign of it breaking down. Time will tell, I guess.
We've never sought treatment for it and we don't plan to - unless it starts to grow again or has big changes in appearance.
Me - Aimee - Spot on liver
Elliot, 9/20/06 - H on back (nearly gone)
Felix, 7/28/08 - H on forehead (in involution)
Taking the 'wait and see' approach, for now.