found a few links/sites that suggest you can do this...
http://www.johnbridge.com/messages/topic/4053-1.html
the issues discussed are: don't tile over an expansion joint, there can be problems with cracks telegraphing through the slab through the tile, and they discuss different options for the morter bed.
sounds like you can do it, but there may be issues, and some suppliers won't guarantee their product in this app.
and another site gave:
"Anyway, I spoke with a mason about how to do it. here is what he said:
1. Be sure that your existing concrete is perfectly clean. A pressure washer should do the job fast and easily.
2. Use concrete rather than mortar to set the stones in. It is harder and more durable than mortar or grout products.
3. Use a trowel to apply a thick enough coat to a limited area so that you push the stones into it and it squishes up to fill the cracks. He said that setting them and trying to fill the cracks aterward is a major problem involving painting all of your rocks with a releasing agent so they don't get covered with the cement. "
and finally:
http://www.homeadditionplus.com/How%...te%20Patio.htm