The long answer is YES, but it's not a black and white issue. I feel comfortable saying it's VERY compliant, but uncertain I can say completely compliant:
1. We specifically addressed a lot of issues in 1.8 that S508 is concerned about here (drill down for details):
2. User-uploaded content may NOT be S508 compliant - there's nothing we can do about that except via education.
3. Moodle is huge with hundreds of pages and thousands of combinations. We've tested these as much as possible and feel pretty comfortable all the pages people are likely to use (99%?). You can do a test yourself of individual pages like this:
4. If you find a page or some component in Moodle that is coming up as not compliant then it's simply a bug and should be filed in the tracker so we can fix it in 1.8.x and beyond. The fact that these reports have dwindled down to nothing is why I feel confident that 1.8 is very compliant.