to be honest I've no idea, but I recon that any soil will hold moisture against the wood and speed up the rotting process..
if the concrete is above / level with the ground and angled away from the post, then any water on it should run off fairly quickly..
in my humble opinion the concrete should be above ground level ! yes concrete is a bit pourus and will hold some water but it is nowhere near as damp as wet soil which will inevitably/eventually cause the wood to act like a wick and soak up water which will in turn make it rot faster. this will not be a short term problem for treated wood but as the wood ages it will become more of an issue. sure this may only make your treated wood last a couple of years longer but in my opinion it is worth it since it costs pennies to do. also remember to make the concrete at an angle which allows the water to run down the post and away to the surrounding soil
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