It's already been requested. Do you really think that not providing that ability was a smart idea? The look and feel of a webmail interface -- the visual gateway into someone performing email tasks and being productive email-wise -- did your UI / Marketing people really think:
"Hey, nobody needs a horizontal split. After all, no other provider does it that way, so no user would ever be use to that and never want it".
Wrong!
Implement a horizontal split view in email
~N