The first thing to remember is that the EPA rating is not a prediction. It's a comparison.
The EPA rating is generated in a lab, not on the road. That's why you can compare one vehicle to another: they were all tested under identical conditions. If automakers apply the test honestly -- I'm talking to you, Hyundai-Kia -- it's the best way to compare vehicles' fuel consumption and cost.
The EPA fuel economy ratings aren't perfect, but they're the only game in town. If hybrid technology outstrips the current test, the EPA and Department of Energy have plenty of PhDs to fix it.