Searching around reveals a few articles and such, but most are very old (circa 2007) and not relevant to the newer version(s) of Mail in OS X Lion. Many of us are very particular about email delivery and want to know if we have messages waiting to go out. Even if you go offline and then compose a new email and hit Send, you will still not see an Outbox (some users note that their Outbox will show up temporarily, but will be gone after restarting Mail).
Newer versions of Mail, by default, don't show your Outbox in your folder list.
