Humans can't be — and aren't — trusted to follow their stated intentions.

This is why you implement systems that prevent humans from doing wrong (either intentionally or unintentionally).

A commenter named Trevor even pointed this out to 37signals in their blog post as to how:

  Did you know that Oracle provides Database Vault. 
  What it all allows you to do is set it up to prevent 
  event DBAs from viewing or modifying data.

  Idea being, DBAs should be able to “administator” the 
  database, but should not be allow to either VIEW or even
  MODIFY customer/employee data (e.g. credit card #, SSN ,
  salary data, etc..)

  There is another product Oracle provides which is called
  Transparent Database Encryption . What it does is encrypt
  your customer data on disk, but then when a database 
  select is issued – it unencrypts the data on the fly 
  without needing to modify your application code.

  Unfortunately, no such products like this exists for MySQL.

  Given the size of your company now and how much 
  sensitive customer data you are now storing, might be 
  worthwhile for you guys to seriously consider using 
  Oracle now.