There is an API to the DB that is easily accessible. All YAF functions available to the forum are available to you as long as you reference. If you know what you're doing, it's easy to work with.
Currently, there are no web services.
I think the reason there aren't many responses is that YAF is open source -- I mean, just download the source and take a look. If you are truly evaluating YAF, take the time to do that step.