My wife and I are building a new house. We more than likely will go with in floor electric radiant heat. And we'll more than likely go with a multiuse tankless water heater or point of use electric water heaters. We figure that since we'll go with electric radiant, we don't see the point in getting natural gas just for hot water. Although we may change our minds, depending on gas fireplaces, gas stove, dryer etc. But having said that, if we went all electric water and main heat (no fireplace, and electric cooking and drying too), with the usual other electrical needs for a house of about 1500 square feet, would 200 amp be enough? We are also in Canada, which might play a part.
EDIT: Also, I forgot, we'll have central air conditioning too... oh and eventually maybe a hot tub? Maybe not...