Heres what I think about Planescape and Ebberon.
Most of what the natives of Ebberon know about the planes is a lie. It is true that Ebberon floats in the Astral Plane for an unknown reason. Thirteen color pools orbit Ebberon. These lead to various planes.
Daanvi = Arcadia
Dal Quor = Region of Dreams
Dollurh = Hades
Fernia = Elemental Plane of Fire
Irian = Demiplane
Kythri = Limbo
Lamannia = Beastlands
Mabar = Demiplane
Risia = Demiplane
Shavarath = Demiplane
Syrania = Demiplane
Thelanis = Aborea
Xoriat = Far Realm
Other planes are considered Remote and are therefore difficult to get to. What do you think?
I'm of the opinion that, like Faerun, Ebberon has been cut off from the rest of the multiverse and its planar structure has been manipulated to suit the ends of those in power. On Faerun, it is clearly the gods, Ao in particular, who are responsible for the partition from the multiverse, but on Eberron it is less clear. Eberron's gods seem to exist, but they aren't obvious about it, and their true natures and motives are less than clear. That said, even if I'm not entirely sure why Eberron would be cut off from the rest of the Multiverse, I have a pretty good idea of how it is.
Eberron is just another Crystal Sphere on the Prime, but the Ring of Syberis and perhaps other anomolous occurences in its wildspace make it totally innavigable by Spelljammers. Access to the planes is just as restricted. There are few or no portals to the Inner or Outer Planes, and travel on Ethereal, Shadow, and Astral is difficult and seemingly impeded. The only thing of note in Eberron's limited cosmology is a string of Astral Demiplanes -- thought constructs floating in the void similar to those Psions make, but on a massive scale.
The apparant purpose of the Astral Demiplanes is to create something akin to surrogate planes for those who have been cut off from the greater multiverse. Basically, I think that Eberron's "planes" were created by the conscious or unconious attempts of those who had been cut off from the greater Mutiverse to create a new set of planes to fill the gap. The Astral is made of belief and ideas, so it was only natural that enough undirected belief would be enough to create that which was believed in.
Basically, Eberron is disconnected from the Multiverse, but surrounded by demi-planes that largely take the place of the true Outer and Inner planes.
As for how someone from Eberron would get to Sigil or vice versa, the Lady's will is not something to be denied, and her portals go everywhere, even (perhaps especially) places where they aren't wanted. Beyond that, Spelljamming, Planeshifting, Astral, Ethereal, and Shadow Travel may be difficult (especially since the Eberronians don't think there's anything to travel to) but are not necessarilly impossible.