UPDATE: A few days after this was originally posted, USPS post office registers were updated nationwide so that they no longer accept gift cards as debit cards for money order purchases.
In bad news for those who use gift cards to purchase money orders at the post office, Miles Per Day is reporting that a memo has gone out regionally to USPS locations advising them to stop accepting gift cards for money orders. Based on the copy of the memo posted at Miles Per Day, it sounds like this is not yet hard-coded into register, but that is on the docket in the not-so-distant future. In the meantime, clerks are being advised to stop the sale when they notice someone is paying with gift cards. Again, this memo is reported to be regional thus far, but it sounds like the end is near for using gift cards to purchase money orders at the USPS. See the post at Miles Per Day for full text of the memo.