If you can’t see the tree layer and your Google Earth user interface is other than English, change the language to US English and the Trees show up in the 3D buildings options http://earth.google.com/support/bin/static.py?page=guide.cs&guide=22358&topic=22360&answer=148181
Today, the German journal Spiegel reported about the tree feature in the latest GoogleEarth version as well, also pointing out the lack of trees for German speaking users. http://www.spiegel.de/netzwelt/gadgets/0,1518,731998,00.html