Well thought Paul! Looks like you just came up with the defacto standard for obfuscating email with CSS. At first glance this works better than all the other methods. The correct copy/paste functionality is upheld, and there is no extra markup. I tested it also, that seems to work well. Not quite sure if I fully trust it since every browser needs a different rule. Time will tell. But seems to be perfect.
As a side note... trying to think of any times no float on span would fail... looks like you can safely do away with the float left (presumable put there to contain the floated :first-letter)... which makes this more usable in the real world.