Edit: this is a very old post but I saw it comes up very high in Google searches.
Not long after this post was made, I began studying permissions in great detail and now have a security guide here on the forums that explains them.
I have also become an app developer since this post and have an app version of this guide with a few small added features (like permission search/show which apps use a permission)
My security guide:
How to be safe, find trusted apps, & avoid viruses
The app version of the guide:
PocketPermissions
(my original post)