Well, the basic problem is as follows: The organic conductors and light emitters can react with oxygen when they are operating, and they will decompose. Many organics are oxygen permeable, so this oxidation happens rapidly unless you have a barrier material - in fact this is the same problem you have with PET (polyester) beer bottles - oxygen comes through and ruins the beer - soft drinks aren't affected by oxygen. It doesnt take much damage to ruin an oled, because the oxidized material acts as a short circuit. You don't have this kind of problem in an LED.
Heat is another issue - organic materials expand a lot more when you heat them compared to semiconductors - this puts stress on the device and can open gaps to let air in.
LED lights are coming and you will like them. The DOE solid state lighting program is making sure of it. It will be a few more years. However, even today, the $50 to $100 LED lights that you can buy right now will _still_ save you money - that is if they last the 50,000 hours that they are supposed to last. The problem is that light bulbs are becoming a capital investment. We will be rethinking our lighting designs for home and office over the next decade or so because of this. The household lamp may go away.
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