I agree with TiagoTiago. And if the receiving addresses are known to belong to lost wallet.dat files then the coins will never be spent and those dirty blocks cannot be pruned.
it would be pretty easy to encode the offending data in the message part of a transaction, and the data (when concatenated together) could be verified as "evil" using a hash set database like the NSRL Software, or the NCMEC database, or HashKeeper.
http://computer-forensics.sans.org/blog/2010/02/22/extracting-known-bad-hashset-from-nsrl/http://works.bepress.com/cgi/viewcontent.cgi?article=1001&context=stephen_hoffmanhttp://en.wikipedia.org/wiki/HashKeeperThis means that you wouldn't need a program to decode it. It would be, as em3rgentOrdr pointed out, an illegal number. Write it in binary to disk, change the filename to jpg and anyone would be able to see it.
hmm. the success of this attack on the network relies on our inability to change the block chain, and the block chain's stability is what drives the trust that we have in the impossibility of double spending.