The Tree is in Helen Putnam Regional Park in Petaluma...
..and it's known locally as The Climbing Tree. It's well known to locals and it's still there. At least it was as of November 2010. I highly doubt it's been destroyed, as it's in the park area. The view has hardly changed, nor the rough grassy area the boys are seen on in the film (depending on time of year).
And to the best of my knowledge, this is the very tree right in the middle, so you can see it's not hard to find in the park.
http://maps.google.co.uk/maps?q=Petaluma,+CA,+United+States&hl=en& amp;ll=38.216967,-122.659371&spn=0.005867,0.009516&sll=53.8006 51,-4.064941&sspn=18.021622,38.979492&oq=petal&hnear=Petal uma,+Sonoma,+California,+United+States&t=h&z=17
Go to Petaluma and see.
"Never underestimate the power of denial."
American Beauty