Okay, if you get this blog remark:
Dude.. I am not much into reading, but somehow I got to read lots of articles on your blog. Its amazing how interesting it is for me to visit you very often.
then you have spam.
Update 2009-12-03: Similarly, if you get
"Good post mate!! Keep 'em flowing!"
then you have spam. I guess comment spammers are going for generic remarks that look like they are paying attention to the blog's content as an alternative to, say, having a genuine interest in the blog's content.