[an unfinished sandbox]
This is the world of all the d20 Modern campaign settings. (In D&D Next, all the d20 Modern, Alternity, Amazing Engine, and other TSR RPGs ought to be recast as D&D settings).
In D&D Earth:
The Imaskari helped build the Pyramids.
Ed Greenwood really met Elminster, Dalamar, and Mordenkainen.
This world is called Laterre by the Averoignians of Mystara. It is their homeworld. Likewise for the McGregors of Klantyre.
In 1986, Mystaran immortals visited NYC and Chicago in IM1: The Immortal Storm
The location of those Ravenloft Dominion novels which are set on Earth.
The setting of The Summerhill Hounds First Quest novel, which follows the adventures of awakened animals as they sail from the Old World to the New World. This novel indicates that in D&D Earth, there is an Isle of Orcus in the middle of the Atlantic Ocean. Mentions deities/Immortals: St. James and the Silver Dog.
This is the homeworld of the Kids from the D&D Cartoon Show.
No proprietary, copyrighted brand (that WotC doesn't own) is included in D&D Earth. So, there are "D&D Earth versions" of all the trademarked franchise brands. Some of these have already been featured in d20 Modern books.
"Cola" brand is the most popular soft drink.
At least in one timeline, the "pre-Gazetteer" Mentzerian/Froidevalian world of Urt from the BECMI boxed sets is the pre-history of D&D Earth--the Jurassic Age to be exact. In the BECMI sets, it's said explicitly that this is so. Mystara, as such, appears to be a different world than Urt, but with the same continental map, but with Mystara having a much smaller diameter. And Urt/D&D Earth isn't hollow. The Masters Set map of Urt is later said to be almost entirely wrong as far as the names and boundaries of the countries. And later materials state that Mystara is much smaller in diameter than Earth, and in Wrath of the Immortals, Earth ("Laterre") is said to lie in an entirely different dimension. This suggests that Urt as described in the BECMI boxed sets is one world (and lies in the past of D&D Earth), and Mystara as depicted in the Gazetteers and other later products is an entirely separate world.
Oerth crossovers with Earth: http://www.canonfire.com/wiki/index.php?title=Earth
D&D Earth has multiple timelines. Whichever Campaign Models are not compatible with each other are in different timelines. All TSR/WotC roleplaying Intellectual Properties which feature Earth are included in some timeline of D&D Earth. The default timeline is Urban Arcana and Age of Adventure. Other timelines may exist as fiction within the Urban Arcana campaign model.
Other timelines of D&D Earth:
Gothic Earth of the Masque of the Red Death is one timeline of D&D Earth.
The non-magical Earth mentioned by Gygax as a parallel of Oerth, Aerth, Yarth, and Uerth, is another timeline of D&D Earth. Only non-spellcasting classes in that timeline. This would be the closest representation of our actual world.
D&D Earth exists in the different Realities (rules universes) too:
In the BECMI Reality, D&D Earth is in the Dimension of Myth, described in the Wrath of the Immortals boxed set
In the 1e Reality, D&D Earth is glimpsed in the pantheons of Deities & Demigods.
In the 2e Reality, it is depicted in the Historical Reference series and Legends & Lore.
In the 3e Reality, there is the Robin Hood mini-setting, and the Olympian and Pharaonic pantheons.
All the setting events ("fluff") from all these iterations, however, exists in all of different Realities. For example, in the 5E Reality, some Mystaran Immortals still traveled to Chicago and New York in 1986 of the D&D Earth of the 5E Reality. It's just that they're portrayed slightly differently though the different rules lenses.