Here is a question. When is the rate figured? Is the FIAT to BTC or vice versa calculated the moment that the deposit is made or when the withdrawal is made? Does the balance flucuate over time based on the current exchange rate?
As far as I know , It's when the Load of the card is made. When you deposit funds using Bitcoin they stay in your account (as bitcoins) but once you load the card they become USD (the exchange rate is done real time I suppose). (Keep in mind that deposit transaction using Bitcoin will only show after having 3 confirmations in the blockchain)
In other words, let us say that I deposited 1 BTC two years back, when the rate was $250 and left it in there until today. Now, I go to make the ATM withdrawal, do I get $250 available or $550?
if you had deposited 1 BTC two years back, it would remain in your wallet in BTC. If you decided to load those BTC to your card now, you would get the current USD/BTC exchange rate - at the time when you select to load your card, i.e., $550 or whatever the current exchange rate is.
Furthermore, if you have USD in your card and feel that BTC is going to rally, you can buy BTC using your USD balance (again, at the current rate, which, by the way, is very good compared to most services), and thus benefit from the increase in BTC value.