I believe that there is an even more important practical issue.
What if someone manages to embed something very much like the EICER string[0] in it? How many people do you think would use the bitcoin client on windows if their AV automatically deleted the blockchain as it downloaded in a misguided attempt to protect them?
Of course, first we have to know if this is possible at all. Does anyone know if there's either a) 20 bytes with a very high AV detection rate or b) some way to embed more than 20 bytes in a row in the block chain?