Apparently, it's beginning to appear in odd places in Los Angeles, but nobody seems to know who's responsible for it.
Radio host Tammy Bruce posted some pictures of this odd creation at her blog Saturday morning (h/t Pamela Geller) leading me to investigate further.
At this point, all I could find on the subject was an April 25 article from Bedlam Magazine:
A poster of Barack Obama in Heath Ledger-style Joker make-up with the legend 'Socialism' beneath it has been popping up recently on surfaces around L.A. It does not appear to be in the same category as the many benign take-offs on the Shepard Fairey 'Hope' poster, such as the one by Australian James Lillis (more of a straightforward spoof that merely substituted a Shepardized image of Heath Ledger as Joker on the Obama poster).