A week ago, the first of a three-part Google update began, and today the second part is beginning. (Note that Jagger3, the third and final part, will begin next week, probably around Wednesday). Jagger 2 can be seen on only a few datacenters at the moment, with 66.102.9.104 being one of them. The theory, based on Googleguy comments, is that this update has kicked in some spam filters, and Jagger 3 will deal with canonicalization or ranking or supplemental issues. Since I did not see any real changes in Jagger 1, I could not comment much on the update, however, with Jagger 2, I am seeing a few changes. Some of my sites are showing some nice upward movement, so while those changes may not stick, it is still a nice positive sign. Many forum posters seem to like the changes, though a few do not, of course.
By the way, PageRank has been recalculated (to solve the MSN PR2 oopsie), so you may see yet another round of Backlink and PR changes today. In fact, one of my sites fell to a PR0 last week, and is now happily back at PR5. So I guess the Great Google determined it really did have a problem - and actually fixed it!
You can read more about this step of the update either at www.mattcutts.com/blog/jagger-2-update-info/ or at www.webmasterworld.com/forum30/31688-22-15.htm.