Jason on 37signals asked:

Up or down? Blogs usually have the latest post on top and the older posts on the bottom... Blog comment sections usually have the latest comment on the bottom and the older comments on top... What is it about quick conversations like comments and chats that favor the latest on the bottom, yet more static conversations — such as blog posts and emails — prefer the newest on top?

Absolutely nothing about quick or slow interaction.

Input submission area is bottom, so the latest is at the bottom for referencing. Same goes for IM. Anything with input. In fact, put the input area on top, and the arrangement will be magically assumed to be latest-on-top.

In traditional email apps, "compose new" is a new window (not relative to the location of inbox), even if it is positioned relatively, rarely any sent mail gets appended into the inbox. In gmail however, the threading is latest-bottom. Bottom, where the input area is.

Without the above requirement, latest-on-top will be the only naturally efficient arrangement. Less code (no auto-scroll down), less effort (for reader to scroll). Hence blogs, inbox and many other things are arranged latest-on-top.