Is there a limit on how many customers/unique card numbers you can have, and is there an inherent cost in gaining a new number (e.g. will accounts be capped at X disposable numbers per Y period).

Once you delete a card number how quickly is it returned to the "pool?" And do you re-assign it to a new user requesting a number?

Do you think third party web-sites will break because a card number is used twice for two different users (e.g. user 1 and user 2 both have the same number on their accounts, because user 1 let it expire, and then user 2 received it and utilised it).

Are you worried that these numbers could be used for fraud or abuse (e.g. unlimited trials).