Google's Matt Cutts has released a number of videos in which he talks about the way Google works.

Google's Matt Cutts has addresses previous and future updates of the Google index in these videos.

GOOGLE FACTS
Google's index is updated every day as Google crawls the web. There pretty much is always an update going on.

Google has algorithms and data pushes that are going out on a less frequent basis. The latest were on 27 June, 28 July and 17 August.

Google's BigDaddy update was a software infrastructure upgrade that finished in February. The BigDaddy update introduced a new way how they crawl the web.

Matt Cutts mentions in the video that another software infrastructure update is on the way. The new update should increase the quality of the search results:

"If we find out that we can improve quality by changing our algorithms or data or infrastructure, or anything else, we’re going to make that change.

The best SEO's in my experience are the ones that can adapt, and that say 'OK, this is the way the algorithms look right now to me, if I want to make a good site that will do well in search engines, this is the direction I want to head in next.'"