We have a gas fired tankless water heater in our new house. It is mounted on the outside of the bathroom wall so we get hot water in the shower after a few seconds. There is a long run (about 40 ft) through the pipes in the slab to the kitchen and it takes well over a minute for water in the kitchen sink to get hot. Is there an instant hot water heater available that will run until the incoming hot water reaches it and then shuts down?
The only way you can do this is to install a storage tank and put a recirc pump on the tank. That would require running a line back from the kitchen, however. More details available if this is feasible for you.
Actually, you could also put another tankless at the kitchen, but thats getting a little redundant and expensive, dontcha think?
I think this tankless stuff is great, but we as americans are not ready to change the way we use our hot water.
EDIT : I probably wouldn't reccomend putting a recirc on your lines under slab anyway. It's hard on the piping, and you're just asking for a leak someday.
I had a long ranch house in California and it seemed like it took hours for hot water to get from the tank hot water heater in the garage to bathrooms on the far end of the house.
I put a large on demand water heater in for the bathrooms and near them. I put a smaller one in place for and near the kitchen sink and dishwasher. Worked out great. Used a lot less energy and water rationing was an issue at times in N California so anything I could do to help that was in my favor.