While I agree that we need to integrate "green" technology in our lives, I don't like the idea of government mandating it. If governement wants to make incentives for people to make a choice, then great. We should have choices, not mandates.
Requiring solar hot water is like saying you can only drive a Hybrid car. No one would stand for that.
With an average solar hot water system you will spend between 10K and 15K for all the components that comprise such a system. Remember that you are not only paying for a similar hot water heating system as is installed in homes already, but on top of that comes the solar equipment, and a larger tank.
Some one mentioned that tanks last 20 years. This is so far from the truth. I inspect resale homes as a profession, and while every once in a while I might see a 20 year old tank, this is an exception. A reasonable expection would be closer to 10 years for the average water heater. There is a reason the manufacturer only warranties the unit for 6 years.