Update: Matt Cutts from Google confirmed that Caffeine is live 50% of the time in the 18.104.22.168 data center. See below for more info.
In short, people are noticing major changes in the search results. Some believe it is a Caffeine index rolling into their search results. Some believe it is just a normal update and not Caffeine related. We do know that Matt Cutts promised not to roll out Caffeine until after the holidays and a roll out at this time would be the worst possible time for online retailers (unless you rank very well in Caffeine.
We also know Google is eventually rolling out Caffeine to a single data center, which can be why many people are noting an update. But again, it might be that these folks are on this single data center or that this is not a Caffeine update. I certainly hope this was not a mistake and Google released the Caffeine update prematurely.
The data center people are talking about now is 22.214.171.124.
Update from Matt Cutts:
My plane landed and I had time to check. 126.96.36.199 as an IP address currently hits the Caffeine data center about 50% of the time, so that's probably what the person at DP was seeing.
I think it's really risky to take a few threads and turn that into a "major Google update." For example, one of the links you pointed to was asking about the rankings for their site webtlk.com. Given links like http://188.8.131.52/search?q=cache:iwkj8P8xoSEJ:teqtonik.com/+http://teqtonik.com/&cd=1&hl=en&ct=clnk&gl=us&client=firefox-a talking about data recovery and pointing to that site, I'm guessing that Google just got better at determining how much weight should be given to links to that site.
So 184.108.40.206 does hit the Caffeine data center more often than other IP addresses, but it's always been the plan that Caffeine would roll out at one data center (no more data centers will get Caffeine until at least January).
But I would avoid generalizing, at least as going by the one report that mentioned a specific site that I looked into. Of course, on a lot of the webmaster forums you can't tell what site they're talking about, so it's hard to do any debugging on what might have happened to individual sites there.