Fiber reinforcement will usually take the place of standard welded wire fabric but will not replace any required rebar.
The plastic underneath is to keep moisture from entering the concrete from below. If you want a dry floor the plastic will help. To insure it does stay dry you should also use a stone base under the slab.
The plastic will also help maintain the moisture level in the concrete as it cures resulting in stronger concrete. A layer of plastic should also be placed on top of the concrete as soon as it is finished and left in place for 30 days for the same reason.
[This message has been edited by Glenn Good (edited September 19, 2003).]
I would suggest you use a stone base and 6-mil polyethylene vapor barrier though to keep moisture out of the floor to help prevent rusting at the base of your equipment.
PS In case anyone is wondering, I recently changed my screen name. It was formerly GlennG.