In the "Clone Wars" Series 6, episode 12 "Destiny", Yoda has a hallucination where he sees a vision of a dying Ashoka within the Jedi Temple. She asks
You cast me out of the Jedi order... tell me, will I still be able to become one with the force when I die?
This suggests that non-Jedi (or Sith?) cannot join the Force. However, in Episode III, Yoda tells Anakin:
Death is a natural part of life. Rejoice for those around you who transform into the Force. Mourn them do not. Miss them do not. Attachment leads to jealously. The shadow of greed, that is.
And in Episode VI, Yoda says
For my ally is the Force, and a powerful ally it is. Life creates it, makes it grow. Its energy surrounds us and binds us. Luminous beings are we, not this crude matter. You must feel the Force around you; here, between you, me, the tree, the rock, everywhere, yes. Even between the land and the ship.
So... does every living thing pass into the Force after death? Also, what the heck happens to things after they die/rejoin with the Force?