Recently was chastised for Bottom posting on a Mailing list, so I thought I'd write a few words about it.
I bottom (or inline post) mostly because I like the email to be a message. You read a message or a letter from top to bottom, from left to right. It wasn't until email clients started top posting (looking at you Outlook/Lotus Notes) that email was written in the top-posting format, forcing you to read an email backwards.
So I looked it up, basically looking at two different information stores.
Wikipedia -- http://en.wikipedia.org/wiki/Posting_style
RFC1855 -- http://www.ietf.org/rfc/rfc1855.txt
These two places will define how to write email and how email should be written, on mailing lists, use groups, or any other email transaction.
The particular part to pay attention to is in RFC1855 --
"- If you are sending a reply to a message or a posting be sure you
summarize the original at the top of the message, or include just
enough text of the original to give a context. This will make
sure readers understand when they start to read your response.
Since NetNews, especially, is proliferated by distributing the
postings from one host to another, it is possible to see a
response to a message before seeing the original. Giving context
helps everyone. But do not include the entire original!"
Summarize the email at the top, and post below it. In other words, bottom-posting is the correct way to write email, as per RFC.