I'm going to feel pretty dumb if this: http://www-cs-faculty.stanford.edu/~uno/news08.html turns out to be a joke that I didn't grok. According to the link, he stopped writing checks in 2008. I remember feeling very disappointed when I read that.
I just got one of these "certificates of deposit" from him a few months ago. It looks like a check. Unless you were planning on cashing these checks, I don't imagine you'd be disappointed with receiving one of these.
It's true. But you have to keep in mind, nobody wants $2.56 from Knuth. The value comes from the fact that one of the most famous and respected computer scientists to ever live has sent you a thank you note for fixing an error that he made in his book.
I once sent him an alleged error report in which I was mistaken. He cordially wrote back and explained why I was wrong. So I sent him a check for $2.56, and he cashed it...
Knuth no longer sends out real checks, but now sends out what look just like checks, written on his pseudo Bank of San Serriffe (pun on sans serif font). There's a picture of the one I received at http://www.arcfn.com/2011/05/my-knuth-reward-check.html - the amount is in hex: 0x$1.00
The parent article describing in detail how to find an error seems like overkill - my strategy was "I was on vacation and I looked really hard for an error".