A thread at WebmasterWorld named Will google ban portals with rotating content? caught my eye. It asks the question, will it hurt or help my rankings if I change content on a page dynamically. Let me explain. I have a page about a particular vacation spot. On this page, I might want to include; weather that is pulled from weather.com, updated movie listings, hotel information, useful resources that rotate in and out based on page reload. Will this hurt your rankings for this page?
There are two ways to look at it. If the page is targeting a competitive keyword phrase, then changing all the content on the page might prove to be detrimental to the successful and long term ranking of that page. One of the major factors of a page ranking well, when speaking about the 'on-page optimization side', is the content on the page. So if you change the content, the keyword phrase mix might not be optimal.
What is recommended, is to leave fairly static content on the page and then bring in dynamic portions of content, to support the other content. Hand write a two paragraph blurb on the page, optimize those paragraphs for the keyword phrase of that page. Then include helpful resources, such as weather, news, movies, bars and your favorite search engine and you should be set. Fresh content is rumored to be loved by the engines, I have my own theory on that - but that is for a different entry.