Gulf Oil Spill Is Already Half Gone.

You're ignoring the role of microbes (bacteria): an estimated 21 million gallons of oil naturally leaks into the Gul per year (and we assume has done so for zillions of years) and there are well adapted microbes that use it as a carbon source. We can also expect them to munch on the most toxic fraction of Corexit, which is light petroleum distillates.

And don't forget that it's the dose that makes the poison. By my worse case calculations from public data (starting point was a New York Post article) the Gulf has 3.5 billion gallons of water per gallon of oil spilled. Obviously it's not equally distributed, but that gives you an idea of the scale of things (e.g. much greater dilution than happened in the Exxon Valdez spill, where a lot of more heavy (and more sour?) oil was spilled in a much smaller and much colder (less microbe action) body of water).

Note that the "naturally ... evaporated" fraction does "just disappear" as far as the sea is concerned, it's in the atmosphere, spread by the four winds, and diluted to the point it'll take really sensitive instruments to detect at worst.

Seriously:

  Light fractions evaporate.
  Medium fractions are eaten by microbes.
  Heaviest fractions become tar balls.

It's not the end of the world or even the Gulf; compare to the 1979-80 Ixtoc I spill by Pemex of about the same size (although spread out over 10 months). Who remembered that spill or the damage it did (limited except for some medium term problems with some populations of crabs and turtles) until this one occurred?