Hi. I can only speak for the UK as I'm not familiar with law in other countries. There is a growing move towards environmentally friendly 'natural' burials to obviate the waste of wood, the use of chemicals such as formaldehyde etc. etc but UK law stipulates a coffin must be used, however, this could be an environmentally and cheap cardboard affair and need not be mahogany with brass trimmings and such. Hope this helps.
Addendum. It seems that some local councils DO allow uncoffined burials so the law may have been changed or at least relaxed. "Uncoffined burial is permitted providing the deceased is suitably wrapped causing no offence to the living or indignity to the deceased."
Hope this helps even more :)