I have been having this problem all day. I pre-ordered the Hunger Games and now can not get it downloaded! I called apple care a couple of times - most times I just sat on hold forever. The one time I got throught to a person they told me it should be resolved soon, but it's been 18 hours and still no Hunger Games.
I was having this exact issue when I was set up to use PayPal as my payment method. I fixed it by -
1. going to my iTunes account and selecting None as my method of payment, then clicking Done
2. confirming this on my account page - which then said Add (or Set up, I can't remember) a Credit Card
3. on the Add a credit card page, I again set up the exact same PayPal account I had before.
4. I was then taken to the PayPal website to confirm that I wanted to use my PayPal account for iTunes purchases, and finally
5. I was taken back to my account page in iTunes, which looked exactly the same as before BUT...
The next time I tried it, my purchase went straight through! (And this was less than 5 minutes after I'd had the "Credit Card processing is temporarily unavailable" message.)
YMMV, but I guess a similar revalidation process would work for credit cards too.