"Y'all" isn't used in formal writing because it's not part of Standard English. However, it's standard for some Englishes. Its inclusion in southern lexicons comes from Scotch-Irish immigrants, who greatly influenced southern dialects in general. "Y'all" also appears in some other regions of the US, as well as in dialects of AAVE. It's also found in some places outside the US, including Alberta, Canada.
Last edited by Eilonwy; 12-12-2012 at 10:14 PM.