UPDATE: Slashdot says its a hoax. But a good one.
Last week Curiosity was able to use its SAM (Sample Analysis at Mars) device to confirm the discovery. A robotic arm with a complex system of Spectral Analysis devices was able to vaporize and identify gasses from the sample, concluding that it is in fact plastic. How plastic formed or ended up on the Martian surface is quite an exciting mystery that sparks many questions. The type of plastic sampled as we know so far can only be formed using petrochemicals, meaning not only that there could possibly be a source of oil on the Red Planet, but that somehow it got turned into plastic. Even more interesting is that oil or petrochemicals used to create this type of plastic are only known to come from ancient fossilized organic materials, such as zooplankton and algae, which geochemical processes convert into oil pointing to the earthshaking evidence that there was once life on mars.
This looks like a fairly credible claim. It also seems to be a followup on stories like this. So maybe not quite sealing the deal, but strongly suggesting, that our solar neighbor once harbored enough life for it to turn into an oil deposit.