There is some discussion in the forums about a potential Google update underway. Keep in mind, the Google results are always in a state of fluctuation, so updates have to be more about how many people are noticing a major shift. Sometimes those shifts are major enough to be considered an update and sometime they are a bit smaller and either impact a large group of sites or represent the onset of a new update coming down the pipe.
An updated WebmasterWorld thread calls this some type of "rollback." These webmasters are noticing "old" data show up in Google, including old cache, old dates and old snippets. A DigitalPoint Forums thread also has conversation on this possible update. The thread is basically not stating what these webmasters are noticing, but rather an argument if there is an update or not - so it is not too constructive.
There is definitely something going on, if it is a rollback, an old index, an onset to a big update or something else, it is something you might want to be on the lookout for.