The update happened about 2 weeks or so ago, many sites that had been alledgedly sandboxed, appeared to have been released. Also many blogs disappearedf rom the SERP's in general, and appeared under news. Many forums moved up, while many directories moved down. Sites that had been top ranking for a long time went out of the top 1000.

It is not fuly known what changs happened, but the general feeling seems to be that it was an update to weaken link mongering and blog abuse, plus that of poor directories.

Thanks for that excellent summary OWG. I read about it in one of the leading SEO forum but not of the post put it as succinctly as you just did.
You are spot on regarding weak directory, one of my weaker directories seem to have disappeared, its not even responding to its own name

OWG was right on the nail there.

In addition, if you remember Update Florida and the bad effect this had on "Good Clean Relevant Sites" plus the bad press that followed, then you may notice some of the sites that were unfairly penalised are emerging again as a result of Allegra.

Put simply it means that what used to be straight forward honest SEO tripped the Google OOP Filter/Penalty as a result of Florida. This meant that thousands of respectable sites which had ranked well for ages were suddenly given a punitive loading which as a result sunk many below the radar.

It is often said that the Google Algorithm uses about 100 parameters to decide Position in the SERPs. Note I said Position which is different to PR. The sum of points scored across these 100 zones (or a combination of some critical ones) can be "TOO HIGH" and trip an OOP. It is the level of this Tripwire that has been tinkered with in a very complex way. In fact too complex for me to explain in this post even if I knew the tweaks in their entirety which I certainly don't.

I hope this adds some clarification and sparks some further informative input from our many knowledgeable members.