Another data point: I took a $500 bump at MSP tonight with booking on a flight tomorrow. I can get that $500 as any of the following:

* Amazon Gift Card that never expires
* American Express Gift Card that expires but doesn't state when. I'm assuming it's in 6 months based on the posts in this thread but can't be sure.
* Best Buy Gift Card that never expires
* Bloomingdale's Gift Card that never expires
* Delta eGift Card that never expires. It can be combined with other DL eGift Cards, with a max of 3 total forms of payment in one transaction, but cannot be combined with vouchers.
* Macy's Gift Card that never expires.
* Marriott Gift Card that never expires.
* Nordstrom Gift Card that never expires.
* Royal Caribbean Gift Card that never expires.
* Target Gift Card that doesn't state whether it expires or not.