The world emits 70 Mmt/day. Per day. And the US emits about 7 Bmt/yr (2006). So this site would only work for about 14 years.
OK, so maybe there are several such sites possible around the US.
But beyond that, it seems to me the chances of an accidental release of the sequestered CO2 could be phenomenal. If the CO2 escapes, it tends to lie low to the ground (being heavier than air) and suffocates people. You might recall Lake Nyos (1986: 1700 people dead, 3500 livestock dead).
Claims that these underground caverns will be safe...forever... something about it just doesn't smell right. Placing nuclear waste deep underground for millenia is bad enough.... but nuclear waste isn't a gas, even though one heavier than air....