In what ways would co-authoring a graduate research paper as an undergraduate help me? I have been working on a graduate level research project, with the bonus of being listed as a co-author if I make a significant contribution (which I likely will). I am still an undergraduate, but have significant experience (professional and educational) in the field.

My question is: Is this a big deal? If I go to grad school in that field, would this allow me to obtain a PhD quicker? 

What if I don't go to grad school, but choose to work in the field, would being listed as co-author on a research project of this level hold enough weight to warrant listing on my resume?
It doesn't exactly "allow you to get a Ph.D quicker". However, you would have more experience in academic writing than if you hadn't contributed to the writing in the paper. Writing a paper isn't just "writing" a report - a lot of analysis, interpretation, and technical work goes into papers and these are skills you want to have as a Ph.D student. Being a co-author on a paper will make your application stand out.

If you go into any job, you can list this as a project that you have worked on. If you are familiar with the contents of the paper, then you can talk about it as if it was another project that you've worked on. Having your name on the paper is verifiable and can look pretty impressive especially if you are familiar with the details.