I do not believe that it is raining oil. This isn't my area of science (at all) but I did take a few chemistry classes in my day...
Many petroleum products are highly volatile, and almost certainly will enter/have entered the water cycle as a result of the spill in the gulf. It wouldn't surprise me at all if certain hydrocarbons and other nasty bits do show up (in very low amounts) in precipitation (though it would more likely fall somewhere east of the spill area along the gulf stream.)
Crude oil, on the other hand, is very heavy and viscous. It doesn't evaporate in bulk (at least under the atmospheric conditions found on this planet). I could maybe see it becoming sufficiently emulsified (think whisking oil and vinegar together to make salad dressing) as it becomes dispersed that very, very tiny droplets might hitch a ride up into the atmosphere on water. But I don't think it would have the opportunity to precipitate and fall to earth recognizably as "oil rain".
I will stop short of saying this is impossible--I'm not qualified to say that--but I definitely think this falls into the category of extraordinary claims that require extraordinary evidence.
More likely, this is a greasy parking lot after a dry spell.