Once a month Google calculates the pagerank of each page and changes the serp (search enigne result pages) for all keywords.
The Google Dance is the time between the start and the end of an update. In that time you get different results on www.google.com, www2.google.com and www3.google.com. Normally the Google Dance takes about 4 days.
I also found a link to a new ODP category for Google with lots of interesting links about Google.
Other interesting information include what Google conciders spam