Ratings appear very quickly, and do not appear to require approval (can't imagine that they would). The reality is that almost no one rates the apps they download, and since "rate on delete" was eliminated with iOS 4, even fewer ratings will appear. (The removal of "rate on delete," where the OS asked you to immediately rate an app when you deleted it, is generally a good thing, since someone deleting it is much more likely to dislike an app than someone keeping it, which resulted in significantly skewed ratings).
Reviews sometimes appear very quickly -- within minutes of submission -- and sometimes take up to a day to appear. Last year there were some significant abuses of the review system that became public, including mass reviews submitted by marketing companies (through individual accounts set up for that purpose), so it's likely that Apple changed their policies at that point. From what I can tell, if an account has a long history of ratings across multiple developers, reviews can appear very quickly, but there seems to be a delay for new accounts or those with few reviews, which may well be approved by humans.
However, as Phillip wrote, the actual process is entirely unknown, and Apple intentionally keeps it that way.
Personally, its never taken more than a few hours before my reviews appeared, and it's usually within minutes.
Edited to add
Just wrote another review and noticed that below the entry area it says,
If a review is deemed inappropriate, it will not be posted to the App Store.
which implies potential review, and after the review is entered it says,
Your review has been successfully submitted. Please note that it may be reviewed and will not appear in the store immediately.
The "it may be reviewed" at least partially supports my "depends on your account" theory.