My take on this is Edge Rails is not what *I* use for development, but what is currently being developed by the core team. Kind of like getting latest from the code repository. You might also call it a development build. Probably loosely analogous to Debian Unstable.
As far as having all the goodies in 2.0, if yr on Edge Rails... you
have them. Hope that answer helps.
"Edge" comes from "bleeding edge," meaning you have the very latest
version of Rails. To say "version," though, is a bit of a misnomer,
as edge never corresponds to any specific version. As Russell
mentioned, if you have edge, you will always have the features of the
latest proper version, but the crucial difference is that you will
usually also have *more* features with edge. This is because the edge
is the main development trunk, meaning the place where all the work on
Rails is actually occurring. Every so often, the core team will go
through the latest changes on the trunk (edge) and select a number of
them to be included in an official release, which, as Ryan mentioned,
then gets created as a branch of the trunk (snapshot) such as 2.0.
In short, official versions of Rails are released as gems. To use
edge, you would need to explicitly export the Rails trunk from the
official code repository. Since this only happens if you
intentionally do it, and since you probably followed some instructions
that had you install the Rails gem, you're most likely not using
edge. More specifically, if all you did was install the Rails 2.0
gem, then you are decidedly not using edge.