Hard wood floors do not conduct radiant heat well, so they usually aren't recommended for that particular heating method. Plus they can warp from the heat. I believe that engineered hardwood floors are less prone to warping, but they still won't conduct the heat well.

Thank you. What do people put on top of radiant heating?

If you want the look of hardwood, take a look at wood-look ceramic tiles. They can be quite beautiful, and tile conducts the heat better than anything I believe. I think carpeting and stone are also reasonably good. But I"m not an expert, so let's see if some of the professionals here have other suggestions. Here's a website that seems to explain the pros and cons of some of the different flooring materials with radiant heat:
http://learn.builddirect.com/flooring-info/heat/radiant-heating/