I've seen some discussion where folks, particularly in Texas, have used the radiant system for cooling by running cool water through the loops. I plan on doing that in the house I'm getting ready to build. The single most area of concern I've seen commented on is humidity, especially with hardwood floors. Since the hydronic system will not dehumidify you need to keep the humidity down with a dehumidifier or, if the climate is dry, by air exchange. I plan to use air exchange, both for humidity control and night time cooling with outside air. Summer humidity in SW Idaho is on the order of 20 to 40% and nights are cool. I'll add a dehumidifier if needed. I'm also building an ICF house so it won't heat up like a frame house will. Makes hydronic cooling a more viable option.

You might find this ASU study helpful with your project.

http://www.design.asu.edu/radiant/02_caseStudies/carefree/latest/concl_obs_01.html

It is recommended that you utilize a air handler system for AC when you are using radiant floor heating. This assures you of domestic hot water (free) in the summer as it takes heat out of your structure and uses the desuperheater to heat the domestic water. Furthermore you have control of the humitidy.

laneelmer - great question.  There is only one way to do infloor (with geo) and that is with hot water.

So; you have two options as far as equipment is concerned.  (1) use a geo unit that make hot water or cold water (depending on the mode it is set to or (2) use a "triple function" geothermal unit (one that can forced air heat or forced air cool or hot water heat).

The triple function units can sometimes do all three modes, but typically they will be undersized for the hot water portion.

I would recomend option (1) - a Water-to-Water heat pump will make cold water in the summer season that you can pump into a cooling coil.  This will give you standard AC.  In the winter season, the heat pump will make hot water, and you can pump that water into the infloor as needed.

check out: http://www.hydroheat.com/product/magnum.htm - this is a typical water-to-water heat pump.