I didn’t realize it, but Laclede has a customer rebate program to get up a $250 rebate on heater or hot water replacement, if it’s deemed efficient. Additionally, for 2006 and 2007 only (currently) you can get a 10% tax credit on efficiency improvements in your home. Mind you will have to pay more for the more efficient units, but the savings will reduce the time of the return on your investment.
Here is a state Energy Efficiency Index for the US which will tell you promotions that local utility companies are offering.
You can get a tax credit up to $500 for improvements, such as window upgrades ($200 max), doors, hot air / water, and simply adding insulation (materials only.)