(Perhaps this should go in Links, but I might get around to writing some more original content here eventually.)

Rant from Simon Wistow about the practice.

RFC 1855, Netiquette Guidelines.

How to get Outlook Express to bottom-quote.  (Relatedly, How to get Outlook Express to send in text.)

A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet?