Although it does not appear to be across the board, many webmasters are seeing some drastic changes in the Google SERPs. I do not see any significant differences in the results for any of the sectors I normally track, but there is enough talk out there to indicate something has been going on. I have been watching the forums carefully, and it seems as though there are three prevailing theories on what might be happening.
Many people think that reciprocal links are being devalued or penalized, or systems such as Link Vault, etc. are causing problems. My gut feeling is that this is a knee-jerk reaction that will prove to be false, but lots of others think differently.
Since there are many reports of big sites suddenly dominating the SERPs, such as large corporations or Amazon, Ebay, and the like, TrustRank may be a factor. Perhaps the TrustRank knob has been turned up a bit.
Quite a few people are noticing that a site: command produces far more pages listed than the site actually has, and that a lot old, supplemental results are now showing up. This could mean that a lot of old data has been folded into the mix, causing the results set to be skewed. If so, I would expect this old data to eventually fall back out of the index, which would have the effect of returning the SERPs to pre-update status.
I cannot personally help determine what is going on, since none of my sites have been affected in any way, either positively or negatively, nor have the results (whether or not they include my sites) shown anything different for the sectors I track. But I thought it may be helpful for those of you seeing these changes to have a quick summary of what others think may be causing the upheavals.