Basically don't spend money :)
Because Knuth sent is a check. I think everyone who receives such a check will treat it as a mistake from TAOCP, install it with the mirror frame, hang on the wall :)
It is estimated that there are not a few people who really will honor the checks of Knuth. After all, Knuth's signature is worth more than $ 2.56 :)
There are several web pages for certificates:
Http://www.kernelthread.com/miscellaneous/dek.html
http://budling.nytud.hu/~sziGetva/etcetera/tex.html
http://truetex.com/knuthchk.htm
http://mate.dm.uba.ar/~aldoc9/misc/concrete.html
