just becuase Mars (and similar planets) are round doesn't mean the Earth is.
Even though I am quite firmly a RE, I must agree here.
To be fair, that point is quite vital to the debate. As humans, we evolve by mutating. We are different from our parents. Aside from the FE/RE debate, there is also the fact that Earth appears to be the only planet with life (so far discovered, presuming that space probes are in fact genuine for the time being). Does that not mean that Earth could be a "mutant planet," and it does actually have scope to be a different shape from the others.
According to current theory, Pluto has a different orbit from the other planets in our solar system (leaving aside the Pluto asteroid/planet debate). That makes it different also does it not?
According to current theory, planets are made up from supernovae, other destroyed planets, asteroids etc. They could in essence be its "parents" (which also brings up the idea that planets are alive - not an uncommon theory, and certainly evident with the idea of vegetation)
Could that not mean the information from its "parent particles" could combine to form this mutant planet - with a balance of gravity and resources enough to support our lives?
However if gravity applies in this Earth, then it is most likely that the Earth will have formed into a ball. Although, keeping along the ideas of the "mutant planet," and the possibility of all these coincidences happening, the gravity could have been pulling things in, but there was very little to pull in above or below it, resulting in the production of a planet that is extremely "wide" yet quite "short" to the point that it looks like a disc.
Comparing it to current theory, there are many planets that have been discovered that are disproportionate in size. Our own Earth according to popular theory suggests that the poles are flat, so the Earth is not a complete sphere. It is possible that this case is more extreme than previously speculated, and that most of the hemispheres are fate, or at least have a very shallow slope.
Going along with this theory, then we must also assume that the Earth can be flat but still have 2 "sides." Therefore it is still possible to go right round the Earth and be back where you started from.
Perhaps Terry Pratchett had the right idea. Maybe we do live on a discworld?