Barring a catastrophe of global proportions, I think it is theoretically possible to achive a 100% (or at least 99.99%) Always On environment, given that the company/service has the necessary infrastructure redundacies, a strategic geographical distribution of hardware, and fail-over processes that are effective and automated.

Unfortunately, sometimes companies make the promise of "little to no down time" without actually spending the money to realistically back it up. I guess the "little" part of the promise is the loop hole as "a little down time" can be a very relative term...