Organic Chemist here engaging in some speculation.

My best guess: Curiosity's SAM module has discovered significant levels of small organic molecules in the Martian soil. Not just any organic molecules, but either byproducts of life - or more exciting, the actual building blocks of life.

This would be strong evidence for present or past life, but would be far from an observation of life itself.

Some commenters have made the point that 'something for the history books' (the quote from the missions's PI) means different things for scientists and the public.

I'm pretty sure the discovery falls into the category of exciting for scientists, not exciting for the general public.

The data in question are coming from SAM, Curiosity's sample analysis tool. It's equipped with six gas chromatography (GC) columns. The output of these columns can be fed into either a mass detector (QMS) or spectrometer (TLS). IMO, none of these tools can detect life itself:

http://ssed.gsfc.nasa.gov/sam/samiam.html

SAM's purpose is specifically to find evidence for organic molecules - molecules based on carbon. The announcement will have something to do with discovery of organic molecules of some kind:

http://www.nasa.gov/mission_pages/msl/news/msl20110118.html

So direct observation of microbes or any other life form will not be the announcement. I'm pretty sure that's the only discovery that would be 'earthshaking' for the public.

What would be earthshaking for me as an organic chemist?

One possibility would be that SAM has discovered known byproducts of life, such as polycyclic aromatic hydrocarbons (PAHs), as was found in the Martian Meteorite studied by the Zare group:

http://www.news.cornell.edu/chronicle/97/4.3.97/Mars_rock.ht...

PAHs can be made through a variety of inanimate processes, so I'd view this discovery as more ho-hum.

Still, it should be remembered that Viking failed to detect the presence of any organic molecules at all. Finding any organic molecules would be a big change in how we view Mars - but not in my view 'one for the history books'.

However, if familiar building blocks of life such as amino acids or carbohydrates were detected, that would indeed fall into the earthshaking category.

Life on Earth uses a selection of about 20 different amino acids. I would consider observation of a similar assortment of amino acids as strong evidence pointing to past or present life on Mars.

Observation of five- and six-carbon carbohydrates, which are generally much more difficult to synthesize than amino acids, would be even stronger evidence for life. It's hard to overestimate how important that discovery would be.

Observation of peptides - amino acids strung together as is done by all known life would probably be the most mind-blowing thing I could imagine SAM discovering. Depending on what specifically was found, it could remove any doubt - Mars has or had life. Very long peptide chains can be synthesized without life, but it's not easy. The exact composition of amino acids in the peptide chain would be critical for the interpretation and would be more time consuming than just, for example, observing a PAH.

The fact that amino acids, peptides, or carbohydrates themselves were detectable today, given that they do degrade over time, could point in the direction of life having been present recently.