Time Out Chicago did a pretty good article on communal etiquette last fall. What it doesn't acknowledge is that it's pretty natural to feel awkward in the communal situation when you're dining solo. Introverts and extroverts aside, human food-sharing is a primal behavior that we've been hardwired to use for bonding to others at every stage of life - from breastfeeding to dating to power-lunching. We use food to connect with other humans to get our needs met.
And so there you are, sitting between two total strangers, reeeally hoping they don't want to bond while you have your way peaceably with a tuna salad. The graceful thing to do is, as Time Out suggests, at least acknowledge the humans around you. Time Out suggests making introductions - as in, they want me to voluntarily exchange names with the entire table. I have never once seen this done. I think a simple hello should suffice. Then, if you're not feeling up for a brisk round of networking, reading is always a good strategy to politely say, "Do not disturb." Just make sure your newspaper doesn't turn into a weapon as you turn pages. Or you could make like Oprah and pony up for a Kindle. As Amazon's CEO Jeff Bezos was super-quick to point out on Jon Stewart, it's like, totally perfect for reading with one hand.