I found this blog to be informative about guilds, the question of which comes up several times in most Permaculture courses. We structure a guild the way a forest or an ecotone (the transition between ecosystems or habitats) or a disturbed sight recently colonized by plants, structures the groupings and layers of plants and animals in its domain. It behooves us to study natural plant communities in our particular bioregions. It is important to explore areas that have been set aside for conservation, backyards, alleys, agricultural edges, etc to discover what plants are functionally supporting one another. We can then mimic this structure in our forest gardens, annual crop beds, edible landscapes, what have you.
More on guilds later:
http://onestraw.wordpress.com/sub-acre-ag/permaculture-guilds-a-primer/
Also, this video about Plants for a Future is very informative for those that wish to explore the edible landscape in more detail. Plants for a Future is a unique resource for us plant freaks, an everything resource:
[youtube=http://www.youtube.com/watch?v=Zqt4zCeGMnk&hl=en_US&fs=1&] and