No, am not referring to the spate of friendly comments by blogdoctor in most of our blogs, but 'the real' spam.
Not sure, since when this feature is available (no release information from Google), but they have enabled 'Word Verification' feature - wherein the commentors would have manually 'enter' the words that they see, created via bitmap. For more details see this article
Ofcourse, you must be well aware by now, that, the goal of blogging is to get many comments, so you can choose to not enable this feature, there by encouraging more visitors.
It is interesting as to how some of the automated-spammers have not gotten intelligent that they started commenting using 'profiles' - by-passing the 'anonymous' commentor option. It now really is a question for the courts to decide, as to whether, visiting someone's site and saying please visit my site is really a spam ????
Tech Rant : - Guess they use the google's last 100 updated blogs to pick up the blog ID and do a text match for 'this blog does not allow anonymous comments' and decide to use either the anonymous option or 'logged on' option - smart huh ! :-D
p.s : If this is not your cup-of-tea, join-in for the masala milk post below :P