The main issue with feeding artificially softened water into a storage hot water heater is that softening water makes the water more conductive. This isn't a big deal, except for the fact that the protective anode in your water heater tank that keeps it from rusting away into oblivion works harder and faster in more conductive water. This is because corrosion is an electrochemical reaction, and there is actually a small galvanic current flowing through the hot water in your tank and back to the sacrificial anode through the tank wall -- the more conductive the water, the larger this current, and the faster the "battery" created by the sacrificial anode, water, and tank wall discharges, eating up the anode.
As a result of this, feeding softened water into most stock hot water heaters will cause them to die rather quickly of rust -- the anode goes first, then once the anode's gone, the tank itself proceeds to rust out. If your tank is so equipped, regular anode changes (you may have to change anodes at half the warranty period or less!) are one way to keep the tank from dying; however, a better solution would be to fit the tank with a powered anode -- this uses wall power to produce a small electric current in the tank that suppresses the corrosion reaction by driving it "backwards" if you will. (Think of the difference between discharging and charging a battery.)
(I have no affiliation with the Water Heater Rescue folks, btw -- they simply do a good job supporting something most people don't think about, and that's how to treat hot water heaters as something other than a throwaway device.)
