I have noticed plastic outlet boxex being used in residents for wall receptacles. Are the acceptable to be used above the ceiling if the box is accessible?

They can be used in all interior locations.

Plastic boxes for wall receptacles and switches CANNOT be used for ceiling fixtures.

Ceiling fixtures must use a box rated for such use.

Look through this catalog:

http://www.carlon.com/Master%20Catalog/Zip%20Boxes_2B1.pdf

Are you asking about a fixture mounting box or a junction box? As TK said, a wall box cannot be used for a ceiling fixture mount as it is not rated for that application. Your question, however, leads us to ask, for what purpose? The Carlon site shows their Handy Box acceptable for use in a ceiling, and can be used as a junction box if it has a cover and is accessible, although the capacity is limited. I have also seen ceiling boxes used as junction boxes in a wall, behind a hanging mirror.