Yorik sent us a tutorial recently on the process he went through creating trees, using nGPlant, for eventual rendering in Blender. And he even has a number of tree models ready and available for download in the .blend file format for use in Blender. Find them here.
The tutorial goes into discussing visually how tree's appear in nature and the laws that can be used to guarantee you create as accurate a model as possible. And the tutorial even goes into the process involved with painting a leaf texture. A good read and its nice to see various Open Source applications working together in a project.
Find the tutorial here.