I'm sure you've heard about it by now.
My guess is that it is methane, or another compelling organic compound.
I'm pretty damn excited. Life on Mars would be a great discovery to live through.
I really hope it is life. Then the more interesting question would be "what is it based on?" If it is not composed of DNA than we have our first chance to study a separate and independent evolutionary phenomenon. If it is DNA than we would have to concede that there is an extended (and weak) Mars-Earth biosphere. It would be too unlikely for Mars to have developed DNA based life independently.
My bet is that it is DNA-based. I see life acting as a sticky opportunist. Once a planet has it on it, it will be very difficult to get it off. Mars was warm and wet for quite some time. Large impacts on either could have exchanged life-carrying matter between the two. A very interesting possibility is that life started on Mars, and it was Earth that was contaminated.
If it turns out that you're right (and I think what your proposed is the most likely scenario), then I would even go as far as saying it is more likely that early Mars seeded early Earth (as opposed to an Earth->Mars seeding). Although we don't know exactly what Mars was like 3.5-4 billion years ago, most evidence suggests that it was more temperate and stable than the early Earth. In potentially the most interesting scenario, both Earth and Mars developed life independently, and Martian DNA-based life was carried to early Earth and completely eradicated whatever early non-DNA-based life had developed on Earth. It would perhaps explain why it is that all life on Earth is DNA-based. When Charles Darwin wrote "On the Origin of Species" it was not yet known whether all life was based on one basic code/template or whether there were multiple codes. Most scientists were surprised to find out that all life really did seem to have one DNA-based common ancestor. So either DNA-based life adapted a transformative mutation that enabled it to outcompete all other complex organic compounds, or DNA-based life was an extrinsic invader that took over the early Earth's biosphere.
Could be a new virus strain we've all been hoping for.
Thanks for the link. "Pass the ketchup", ha. Seems like methane would be a safe bet considering the past sample.