So how far south do you have to live to have a net zero utility bill with a setup and investment similar to yours?

Deceptively complicated question, Tony. 

Zero bill ($) and zero net energy (KWh or Btu) {or for that matter, zero embodied energy, zero CO2, zero "footprint"} are different metrics, considering different site-specific factors - some physical, most socio-economical.  Throw in per-capita factor and/or "lifestyle" (ie: energy usage) and any simple answers are further muddied...

Just limiting the discussion to PV generated electricity (not my other thermal sys, capturing heat) - moving closer to the equator generally (but not necessarily/directly) yields more energy. However, PV (silicon based) will de-rate (make less energy from optimal) as the ambient temperature exceeds about 70degF...

Example, my rig is has about the same potential as an identical array in Miami - we share similar insolation (radiation from sun per sq meter), they have better angle and less clouds/inclement weather, but their PV overheats & de-rates more of the time. Overall, the two systems will make similar energy/yr.  But for simplicity sake, moving closer to the equator, and optimizing accordingly, will do better...

Much more of a factor, though, are different utility rates, monthly grid connection fees, and/or time-of-use rates, local incentives, etc -  having a very big impact on $/yr. 

Take my same system and put it in the right county of California (with much higher utility rates, and peak-use/stepped rate billing) - and you can triple the $/yr saved, ROI, payback, etc.

My PV setup/location is making about 60-65% of the electricity I use/yr.  I also sell the REC's generated (renew. energy certificates - the "name rights" to the green energy produced) for about the same $/yr as the energy $ I save.  Which is how my electric bill (including monthly connection fees, taxes, delivery fees) is near zero. (Or I could cut my electrical use further...)

Battery based off-grid systems, make this much more obvious - as there is no buy/sell interaction with the grid.  But they have to be oversized to meet the minimal needs of the worst case senario (winter), and have nowhere to "sell" extra energy made in the summer -  so the metrics are much different.  Usually, the key factor for "viability" is comparing/competing against the high cost to even tie into (trench or pole) the utility grid in the first place..

As for natural gas (ie: heating here in IL), I'd have to scrap the building entirely and start over before getting close to zeroing out my gas bill $...  

Hope this helps...!

-Jim

I had no idea about the panels getting too hot! (de-rating)

Thanks for the in-depth answer, of course, lacking the background, I can't return the favor.

what on the horizon of solar energy has you the most excited for the everyday consumer? 

The more we play with this stuff (PV), the more it evolves and the cheaper it gets. It's been a continuous downward slope since PV was invented about 55years ago.  Get it down to about $1/watt (installed capacity) and it'll be cost competitive with coal/nuke from the get-go (never mind all externalities/subsidies/legacy-advantages routinely disregarded with those forms).  Then it's literally power for/by the people - centralized or distributed.

Thin films that are flexible (and require only a very thin coating of silicon) and can be made with the same infrastructure as newspaper ink-jet printing presses. Nano-tubes, multi-layers, optics and other techniques that present a 3D profile for more effective capture of the sun's light energy. There is also experimentation with non-silicon based materials that exhibit the photovoltaic (PV) effect - some have promise to be less heat sensitive (not that that inherent property is really holding silicon PV back).

Also interesting is that we're at/crossed the tipping point for poly silicon production (the main ingredient of PV solar collectors) - more now goes to PV collectors than computer chips.  Which has the savvy silicon producers attention/incentive, as the way to keep their field of enterprise on top of the game, so to speak...  When we add their voice/lobby/expertise to influence the legislature, it might finally break-out in this country...

-Jim