If you have Prime shipping, you can use it as normal, or on those items you really don't care about getting really quickly you can now chose to receive a $1 Amazon credit that can be used for purchasing books and ebooks.

Here.

Not a lot, but if you really don't care about getting an ironing board cover in a couple of days, like me, maybe the $1 is worth waiting.

Cool.  An incentive to check "no rush". 

That will be nice.  I really like the free shipping with Prime, but usually I don't "need" the item in two days.  Amazon and UPS had a lot of problems last year with getting packages delivered in two days during the Christmas season, this is probably a way to help ease the load.

I just tried this.  I bought a christmas gift that I don't need anytime soon, and picked slower shipping.  It said I would receive a $1 book credit, but instead I was sent a $1 MP3 credit.  I'm sure I can use it, but frankly I'd rather have the book credit.  Anyone else run into this?

No, mine just said something about adding it to my account. Haven't tried to use it yet.