2 questions:
1.) I think it does. MVC gives you full control over the HTML that is being rendered, which makes it easier to be fully 508 compliant.
2.) I'm working on a 508 compliant solution for the government that uses Sitecore as the CMS. It's a fantastic product all around and great in terms of 508 compliance. In addition to giving you full control over the HTML that is being rendered, it also has some neat built-in 508 features. For example, if people add an image to an item, the system will warn them if they forget to add alt-text. You can also add your own validation rules to the system.
Highly recommended, but not excactly cheap.