Today, WoW.com can report that these mobs are again visible in Stormwind on some servers. We've verified that when you view nameplates on mobs (by pressing the V key) and look up around the Stormwind bank, you can see the [DND] Shaker mob, as pictured above. The Shaker mobs are also present in the trade district. When you are able to view the nameplates, you can't view the actual mob itself -- they are invisible / have no form.
We're not sure how they work. We think they appear at random intervals, as sometimes we can see them and other times we can't. We also are guessing that they might only be able to be seen on some specific servers -- we know they're on Sisters of Elune and Ner'zhul.
However it's important to make very clear that other than their appearance and function, we don't know for sure how they appear.
We're interested in what others are seeing, on what servers, and under what conditions the mobs are seen. So far our staff has only been able to verify that these mobs are present in Stormwind, however we wouldn't be surprised if they're just intermittently visible in other cities.
We know that in Cataclysm Deathwing is going to come back and destroy a lot of old world Azeroth. Entire cities can change, and lands will be torn asunder. These earthquakes appear to be part of the lead up to the event, with Deathwing's return causing the very land to shake in fear.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.