I agree with Jay for the most part, but I get the feeling that a little clarification on how moist air behaves might help.
For those who may not know, reletive humidity (RH) is the ratio of how much water air is holding to how much it can hold at a given temperature (and pressure, but we can ignore that). As air temperature increases, it's ability to hold water increases and vice versa. As the air contacts the concrete, it's temperature falls and it's RH rises until it reaches 100% and you get condensation. From this it should be apparent that there are only two things that can be done: lower the RH of the air, or raise the temperature of the concrete; neither of which is practical without serious effort. Enclosing the porch as Jay suggests and dehumidfying is a possibility, assuming you want a room rather than a porch. If the slab had had insulation placed under it, there probably wouldn't be a problem (real helpful huh?). Since raising the slab temp. isn't practical, covering it with some sort of air tight insualting material and new flooring is about the only thing left. Good luck with it. I would be tempted to investigate the insulating properties of something like Nobleseal (on which you can set tile). I doubt that it's very high, but it might be just enough.
FWIW, the condensation process is highly predictable. If you mesaure the slab temp. you can predict at what RH condensation will occur (and vice versa) with the aid of a psychrometric chart. A chart and a small manual on how to use it is available for a few dollars at www.acca.org if anyone is intersted.