Thanks to "Sayhey" on the MacRumors site for these Top 20 largest metro areas without an Apple store, which could be likely new areas to get a store.

1 - New Orleans, Louisiana
2 - Greensboro/Winston-Salem, North Carolina
3 - Greenville/Spartanburg, South Carolina
4 - Dayton, Ohio
5 - Stockton, California
6 - Birmingham, Alabama
7 - El Paso, Texas
7 - Bakersfield, California
8 - Allentown/Bethlehem, Pennsylvania
9 - Harrisburg/ Lebanon, Pennsylvania
10 - Scranton/ Wilkes-Barre, Pennsylvania
11 - Toledo, Ohio
12 - Baton Rouge, Louisiana
13 - Youngstown/Warren, Ohio
14 - Springfield, Massachusetts
15 - Sarasota/Bradenton, Florida
16 - Little Rock, Arkansas