Gluing Hardwood Floors?
Yes, hardwood is often glued directly to concrete slab. It's a fairly common installation.

That said, gluing down solid hardwood planks is more problematic and trouble-prone because the solid planks do not tolerate uneven floor very well.  

Engineered hardwood often works better for a glue-down application.  Engineered hardwood is manufactured from layers of material laminated together, topped with a veneer of actual hardwood.  The layers are chosen to allow the flooring to better tolerate environmental conditions, such as moisture, better than solid planks.  Manufacturers also often cut slots on the back of the hardwood which allow it to flex and absorb the unevenness in a concrete slab.