Nature will eventually clean up most of the spill, just like it did with the Exxon Valdez and Ixtoc spills. Sometimes I think the environmentalists want BP to fail at containing the leaks. They just want to bring down the big corporations, right or wrong, and it's been 3 weeks of sensationalism without thick black oily crude washing up on the beaches; they need a pay off for all the hype.
Getting back on topic, I think nature can do a lot of the cleaning but I think it would be wrong for BP, the gov't, etc not to help it along, as long as we do our best not to do any more harm. We (humans) created the mess, we should do our best to clean it up. Lately there's been a lot of criticism of dispersants, but honestly I think not using them would create an even bigger problem. Ideally if the oil could be collected (containment dome, top hat, etc) or stopped that would be great. But dispersing the oil to the sea floor where most of the bacteria that eats away at the oil that naturally seeps from the sea floor is better than letting it wash up on the beaches or move inland where it would be a lot more difficult to clean up.