The hardwoods will actually retain more heat than the carpet will.
On sunny winter days passively heating the hard woods will cut down on heating cost and this type of radiant heating is very comfortable.
You can also run radiant electric or hydro heating systems under hardwood floors.
For more information:
Read "I'd like to put in hardwood floors and I'm considering bamboo. What do I need to consider?" a Q&A answered by Joel Hirshberg.