My first thought was of them being pawned off to the poor; then, that they are too smart to buy them so they would go to the scrap yard but considering the EPA it wouldn't be cost efficient; therefore, I am left with the image of dead battery cars lining the roadsides.
As things get worse its possible, though like most subsidies, the incentives to buy hybrids helped rich people buy them, not the "poor." (the poor will now buy an even more expensive used vehicle now that cash for clunkers took so many off the road.) But as the batteries age, and these hybrid owners are suddenly stuck with a $6000 bill to replace the batteries, and probably another $1000 in environmentally safe disposal fees, they might realize that little Green Hybrid will never save them a dime, even if gas prices triple. Plus the range/gas mileage will decrease over time. You know that laptop battery you had that used to last 5 hours when it was new but now lasts 45 minutes if you are lucky? Yeah. But now its your car, and you can't plug it in while you are driving.. On the Gas powered hybrids, you just end up burning gas- I would expect some owners will start removing the batteries because that will reduce the weight of the vehicle and better the mileage. On the Plug-ins they will be turning off the radio and AC just to get a little farther. (Yeah, I would like to come over, but I won't make it tonight, using the headlamps consume too much juice) . A plug in is going to require you to buy the equivalent of a cheap used car every 7-10 years. And don't even get me started on how temperature changes Battery performance. Ever had a problem getting your car started on a cold day with on old battery? Imagine when that battery doesn't just have to turn the starter to unlock the chemically stored energy in gasoline, but has to actually make the car go! Wait till a plug in hybrid is filling up the right hand lane at 20 mph on a cold day. Keep in mind discharge matters too. Most batteries can't use all of the electricity they store. Discharge a regular car battery more than 20% and you start to destroy the plates. Discharge a "deep cycle" more than 50% the same thing happens. The best batts I found ( for Solar anyway) can be discharged to 80% - 2000 times. 2000 isn't a lot if you do that every day. If you have a 10 minute commute a plug in is fine. If you go farther, and go past what those batts can comfortably discharge, then you burn them up faster. Somehow, I haven't seen that in the sales literature.
There is some promising tech ( ultra capacitors etc) that might make (shorter term) electrical energy storage more viable, but right now, the chemical reactions we rely on in batteries wear out the batteries themselves - the only way to make them last longer is to build them bigger, making them heavier and thus requiring more power to even make the vehicle go. Its a loosing proposition all round because a internal combustion engine, properly maintained will last 100,000 miles or , and even then can be rebuilt and used for another 60-100K for about the same sum you will be sinking into your hybrid every 7-12. The Gas powered aren't so bad, because they can ensure your batts are not over discharged. ( I guess a plug in can too, but just not making that capacity available to you and leaving you stranded) but even if kept above the optimum discharge level, most bats won't handle more than 10 years of service.