That's so obvious now you mention it :-)
Any spammy link created now should simply be discarded, it either is an attack or it is a fool. Either way ignore
1. I already have spam links pointing to my site. It hurts me so I clean up the spam and make the world a little better.
2. I add spam links to my competitor who previous had no spammy links. Google notices the dates on the links and does not alter the PageRank of my competitor. The world is not made a little bit worse. I stop doing it.
So as long as there is some clear cut of date for spammy links (and hopefully some definition) then it seems workable. Any black hat SEO will know no to bother adding post-Penguin/panda links - for good or ill.
