I'd be interested to hear exactly what has changed since 1995 that
invalidates the original reasoning.
I'm not sure how much has been said so far (archives don't seem to be keeping
up), but Gmail does a few things to make top-posting liveable, almost
preferred. It's about the only client I know that does this, though.
First, it's pretty thoroughly threaded. Even if my inbox is empty, when a new
mail comes in, it'll show it as a whole conversation.
Second, quotes are often hidden by default. They turn into a little JavaScript
link that says "Show quoted text" or something similar.
The result of those two is that it's usually more convenient to drill down in
the "conversation" -- meaning looking at actual, previous messages -- than to
look at a quote. This is because the actual messages will preserve formatting
(links, images, etc), and will also have the widgets associated with
metadata -- I can reply to that particular message, or push a "chat" button
to open an IM window with the person who wrote it, etc etc.
However, having the entire conversation in every message, even if they're all
hidden, has the nice side effect that I can be included in a conversation at
any point and have the entire history of it readily available. This is mostly
useful for business -- someone asks my boss about doing something, he replies
with comments and adds me to the CC list. I have the full context because of
that quote, with no additional effort from anyone.
In communities centered around gmail (Google Groups), and in places I don't
know of a convention, I'll usually top-post in cases like this -- where
there's not a huge amount of correlation between what I say and what's been
said, but it might be useful to have that history. It's also useful in
replies to individuals -- why should I read through everything I just wrote
in order to get to the actual message?
But in cases where it's appropriate (and in communities where it's requested),
I'll bottom-post and, occasionally, middle-post.
Of course, I probably lose any etiquette points for beating a dead horse...