Can I convert a radiator to radiant floor heating in a room?
You may be able to use the existing radiator piping to some extent, but a number of other changes will be required:


Lower water temperature. Because the radiant heating runs under your entire floor, it is typically run at a temperature similar to a hot summer day (around 85 degrees Fahrenheit). Cast iron radiators are usually run at around 150 degrees F, which would be painfully hot to stand on. So if you are keeping any radiators, your system will need to support running at multiple temperatures. According to this piping schematic from hydronicpros.com the device that can both control your boiler and mix water for a lower-temperature section is called a "variable speed pump mixing control".
System balance. Changing the way a room is heated may take it out of balance with the rest of your home, leading it to get too hot or too cold. You'll want to get a professional to assess whether this will require additional thermostats / control modules, or whether there's a simpler way to balance things.
Under-floor access. For hot water radiant heat, you'll need to install PEX on the underside of the subfloor. I don't think you can do that without access from underneath. Probably not a problem for your kitchen but may add some work for the upstairs bath. Correction 2 Feb 2012: Actually, there are hydronic systems that can be installed above an original subfloor (inside a newly poured cement layer, under tile). These may cost more and would likely change the level of your floor, but are worth considering.


One alternative you may want to consider is electric radiant heating. Electric radiant is available in mats that are layered with thinset between the subfloor and tile. The mats can be custom made to the shape of your room, e.g. to flow around a kitchen island. The system will come with a temperature sensor and control that is typically mounted in a wall box next to the light switches for the room. In this case, you'd simply remove the radiators, and use electric heat for those rooms instead.