Effective March 29, 2009 it became mandatory that you comply with New Design Layouts on Flat Size Mail or you lose your automation postage discounts.

This is part of the United States Postal Service's New Flats Sequencing Strategy

I'm going to boil down the ridiculous amount of Postalease trying to explain what they want
so here it is in a nutshell:

Do not place the area for addressing in the bottom part of the mail piece. They want the address information as far to the top of the piece as possible. Most of you have been designing your Flat-Size mail pieces with the address clear area along the bottom. No More! The address area must be in the top half of the mail piece and preferably as close to the top edge of the piece as possible.

Download and watch this Microsoft® PowerPoint Presentation
created by the United States Postal Service®

Unenclosed Flat Addressing Example

Flats Addressing Tools & Resources

Addressing Fact Sheets
First-Class Flats
Periodical Flats
Standard Mail Flats
Package Services

Poster [download and print]
New Address Standards for Flat-Size Mail