I know this is definitely a controversial subject based on so many being against what happens after visiting modern Earth, or the idea it'll somehow break certain themes. But many fantasy stories do have some sort of interaction with modern day Earth I can think of His Dark Materials and the Chronicles of Amber (maybe a bad example as stuff from Earth was used to change that balance of those worlds) off the top of my head. It does come up as a common enough place in stories where there are many worlds.
But even if the campaign does cross-over with Earth, it doesn't have to be modern day Earth, it could be Earth of the past like in the 1500's or even 1800's. Or it could be near-future Zombie Apocalypse Earth, or even my idea of Urban Arcana-Masque of the Red Death-Dark Matter Combined Earth.
And let's not discount alternate history Earths like ones where the Confederate-Mexican Alliance fought World War II against the Polish Empire over North Africa. Or one where the United States was founded as a colony by China (rather than Britain) and rebelled against the Ming dynasty establishing San Fransisco as the capital.
So what are your ideas on such worlds?
I wouldn't want Earth (or alternate Earths) to be involved in the Planscape 'core' material, but I don't see any reason individual DMs couldn't put NPCs (or PCs) from Earth in their games.
BoGr Guide to Missile Combat:
1) Equip a bow or crossbow.
2) Roll a natural 1 on d20.
3) ?????
4) Profit!