Working out the final designs of our house, using ICF walls.  The site slopes to the back for a full walkout.  The garage is a 3 car 'L' shaped (only 2 doors, single stall and a double deep stall to be used for shop space).  Since the deep part of the garage will be at the back of the house, I basically have 2 options...build an 8' retaining wall and fill for a slab on grade or extend my frost walls down around the garage another 5 feet and keep the back area excavated and use either cast in place floor or precast planks.  The size of the room would only be approximately 10x16.  I do not intend to condition it, but rather have a single garage door access to the backyard for lawn mower storage and other outdoor items. 
I have used prestressed hollow cores on commercial projects, and with how small the spans would be this would not be an issue without costing too much.  However does anyone have any idea of how it may compared to cast in place reinforced slab instead of the precast plank options?

Have you considered Lite-Deck?  I got a quote for $4/sq ft for the materials (foam and steel stud stiffeners).  This doesn't include the concrete or rebar.  The span tables show it will easily span 10' with #4 or #5 rebar.

I ended up deciding to use wood floor trusses in my home rather than Lite-Deck, so I have no first hand experience with it.

http://www.litedeck.com/about.html

Lite-Deck and Insul-Deck work well but will require shoring until the concrete gains enough strength to be self supporting. A product such as OP-deck will require a lot less shoring.

litedeck was my first thought keeping the ICF alive, however I would need to drywall or similar the bottom for a fire rating in the exposed storage area below. Not sure how well that would hold up in a non-conditioned possible damp environment...$4 a sqft is not bad though, and I would only need a single shoring beam down the center.  Keeping the shoring in place is not an issue at all since it will be inside the future storage space below.  I would most likely span it crosswise, so only needing about a 10' span.

arkie-  any idea on the cost of OP-deck?

i got a price for hollow core planks, 8" thick. delivered would run around $11.20 /sqft. Plus they would need set by a lift ($700). I think insul-deck is the way to go here.

Is it really cheaper to use foam for a form when the insulation is a negative (needs to be covered)? Seems like traditional formed, cast in place concrete would be cheaper.

my Litedeck rep came back with $3.25sqft for the floor forms.  It would probably be cheaper to form it with plywood, however this space could be used as a heated shop, so an insulated floor may not be a bad idea.  Still, the litedeck, poured, is pushing half the price of a hollow plank.  Also something we can pour with the garage floor as well saving delivery fees.