A few weeks ago, the New York Times ran a story called "In Suburban Schools, an Alternative to AP." The article chronicled the growing interest in dual-enrollment programs, in which high school students take classes that follow college syllabi (and often earn college credits).
The New York Times billed these courses as two things: a cure for senioritis, and an alternative to the "high-pressure" pace of the AP, and its high-stakes end-of-year tests.
I think the first is definitely true. Once the college admissions process is over for the most selective schools by the second semester, people definitely start to slow down. Rather than take easy classes, dual enrollment lets students try out what they'll be doing during the next four years. Dual-enrollment classes potentially expand the curriculum that is available, and by giving kids a taste of college level work may sharpen their study skills.
But I am not sure that these courses are quite equivalent to the AP. For starters, while the AP classes are designed to get you credit (or passed out of courses) at even the most selective universities, many dual-enrollment programs use local community colleges' courses. That doesn't mean they're not good, but Harvard may not recognize these credits. And second, the end-of-the-year high stakes exam is the main reason I like the Advanced Placement courses. They are completely standardized across the country. A 5 in Boise means the same as a 5 in Tampa and New York City. You won't get a high score just because you are the smartest kid in your school. These are hard tests, there is an absolute standard, and if you don't know the material, you will not do well. If you do, though, you know that score means something.
Much in education is incredibly fuzzy. We have no idea if an A in calculus one place means the same as an A elsewhere. We don't even know that the same material is covered. Many gifted kids earn As for no better reason than that the teacher in a regular school has to aim the material to the middle of the class, which makes the material too easy for someone on the high end of the bell curve. A national, difficult, standardized test removes these temptations.