Let me start by saying, Google asked me nicely not to ask them on record if there was an update. So I am not going to ask Google from now on.
But based on the chatter I have seen over the weekend, it seems like there was some sort of update.
Here are complaint posts from September 20th through today:
Seeing significant shift here in the UK, biggest in a long time. Affecting all verticals I watch.
30% drop in UK referrals in the last 24 hours (and slightly more from USA), but I'd have to to look at the stats in a lot more detail to say how far off normal that is.
Conversions? My Inbox is empty, and the phone doesn't ring.
Did something happen today? My traffic has taken a nose dive!
Now, looking at Mozcast, SERPs.com, Search Metrics and Algoroo, they don't show the same thing. Mozcast shows major changes on September 17th, SERPS.com shows major changes on the 18th , SERP Metrics shows changes on the 19th and Algoroo shows them on the 20th. I've never seen all four show significantly different dates for an update based on the chatter I've seen.
Some can be based on UI changes, universal search changes, geo-based changes, algorithmic changes or something else.
So it is hard to nail this one down.
Did you see an update this weekend? If so, when did it start?
Forum discussion at WebmasterWorld.
Update: I have sources that tell me that Google has hit two large link networks over this time period.
Update 2: Here is the post on the link network targeted by Google.