Actually, the concern is more what Groovounet was suggesting: that ATI might pretend that the entire HD 5xxx line may support 4.0 and silently convert doubles to floats.

Without conformance tests, what would there be to stop such a thing? Indeed, I seem to recall something similar happening at the 2.0 transition, when unrestricted NPOT support was required by the specification. Some hardware that couldn't actually support unrestricted NPOTs would still advertise GL 2.0, but silently break if you used NPOTs.