I should have been more specific, I was in a rush =)
The original us-east-1b has chronic capacity issues, meaning that it is always at or near its capacity. AWS refuses to sell instance reservations for this AZ, and it's often difficult/impossible to launch new instances in this zone.
Keep in mind that AZs are remapped for all accounts newer than a certain point in time (say, 1 yr ago). Your 1b may not be my 1b.
