According to the recent Motorweek article comparing hybrid and non-hybrid versions of the Tahoe (BTW, excellent article. They also compared the Civic Hybrid with the Civic EX, and a MB E-class BlueTec with the MB E-class):
"The SEMTECH emissions test shows the E85 Tahoe is cleaner than gas overall. But in the critical greenhouse gas area highway CO2 emissions the hybrid was markedly better despite its bigger 6.0 liter V8."
Markedly better CO2 emissions.
Motorweek also said that the Hybrid Tahoe was about $4000 more than a similarly-equipped "regular" Tahoe (these are very loaded vehicles, $51K and $47K, I think).
Let's do the math:
12,000 annual miles at 14 MPG = 857.14 gallons consumed a year
12,000 annual miles at 18 MPG = 666.67 gallons consumed a year (or 190.47 gallons less)
Over 5 years, that's 952.35 gallons less gas consumed for the Hybrid Tahoe.
Over the next 5 years (60,000 miles) at an average gas price per gallon of:
$3.50, that's $3333 savings in gas (I think $3.50 a gallon will prove too low as a 5-yr avg)
$4.00, that's $3809 savings in gas (this may prove too low, too)
$4.50, that's $4285 savings in gas
$5.00, that's $4762 savings in gas
With the average gas price over the next 5 years at $4.25, the Hybrid Tahoe breaks even in cost with the "regular" Tahoe in just under 60,000 miles. 5 years or so for your average 12,000 mile a year driver.
Even if gas prices stay at the current $3.50 a gallon (not likely), the break-even is only another year. So 6 years (72,000 miles).
And you are sending much less money over to the Middle East (notice my by-line

), and more to GM (and boy do they ever need it).
Now, if your driving is
all highway, the difference in highway MPG is not much at all, and it would take a
very long time to recover the cost difference.
My situation is similar to this, in that the vast majority of my driving would be considered highway. So, I probably would be intrigued by, but not buy, a Tahoe Hybrid for my commute.