Almost by definition, any RPG set on Earth will be an Alternate Earth. The key question is how much variation is necessary to support the campaign.
Well yeah.

The amount of variation I'm thinking about is along the lines of this:
1. That is magic and the supernatural always having always existed to some degree and is common knowledge.
2. Map changes. Well, to a degree this could be easy, but then again it could be the most difficult aspect of tweaking earth (since it also reaches into history and politics).
For instance, New Hope City replaces Long Beach California and it adsorbed several of the nearby towns (including San Pedro). At the same time, Los Angeles failed to annex much of the LA Basin, and ends up being land locked. As a result, the freeway set up would change to reflect the decrease in Los Angeles' importance and LAX would be moved. Now, the big question is, what happened during the LA Water-Wars? That actually is a big important question since LA essentially screwed over much of the South-West and central California as they tried to suck up as much water using their influence and oil-based power.
When I was thinking about Steampunk, one thought that crossed my mind was what if different human cultures didn't have the tendency towards national unification and consolidation? In this case, America would have been a Confederacy, but what about Europe? Would the Europeans have fought the Napoleonic wars or even WWI and WWII? If WWI and WWII happened, would Europe eventually ended up with the current map? In a way, I thought this would make a more interesting idea/setting then the standard super-hero-comic version of earth. However, its also very hard to contemplate.