It seems like every few days there is a new search engine patent out there. Just yesterday a Yahoo! patent was discovered which appears to indicate they have some sort of semantic algorithm they are or soon will be using, in addition to other new ranking factors. Now I haven't read the whole document yet but the abstract seems to indicate that.
What recently occurred to me, however, is a section of the now famous Google patent which was published at the end of March this year. There was a section in it that I read back then that kinda settled in my brain for a couple weeks until a light bulb went on and I realized what the implications of it could be. If you don't already know, that patent looks at many factors when determining rank including age (of both pages and links) how much churn there is in relation to the other sites around yours and so on.