Storing Credentials Securely
dryicerx: I am not sure how mint does it, but several ways to store highly secure data is since you asked for it  * Boxes have no direct internet connection
  * Spread the Ciphertext, Decryption Keys, and Algorithm
    across multiple yet different machines (each with unique
    distributions/operation systems and data storage 
    solution). 
  * Instead of a global salt, use a separate salt per each 
    user (login name / account creation time)
  * Generate the encryption/decryption keys for storing the 
    credentials using a newhash(login+password) and use it 
    only when the user logs in (this hash is never stored,
    it's hashed, generate the encryption/decryption keys, 
    decrypt the data, use the data, and destroyed). So this 
    key only awailable for a short period of time when the 
    user logs in.  
  * Use something strong to encrypt like PGP as the base 
    and roll-your-own obscure one on top of it (which is a
    non-standard algorithm)

There will always be holes and weaknesses, it's just making it really really hard, never impossible.