There are two official links I'm aware of on this topic
http://www.dean.usma.edu/AP/
[URL="http://www.dean.usma.edu/departments/math/courses/ma255/acmp_info_page.htm"]http://www.dean.usma.edu/departments/math/courses/ma255/acmp_info_page.htm[/URL]
And also this post from a cadet:
http://www.serviceacademyforums.com/showpost.php?p=191835&postcount=7
The short version: AP's scores are used by faculty as one source of data in your validation decisions. 4+ appears to be where they start to help in most cases, but even 3's are considered for some. And for a few classes, it appears that very high AP scores (5) can result in direct validation.
You will still have to take a full load, you just get to take classes required later or in some cases advanced tracks, etc.
But read the websites as they are quite specific. Remember, all of this is subject to change, but previous years should give you an idea of what to expect.