It's not like a rabbi stands there and blesses each canister. He certifies the plant -- which probably means ensuring that the equipment used to bottle the CO2 isn't also used for dairy or beer (yeast isn't kosher for passover), and that the equipment isn't contaminated with any pork (some industrial process, not that many any more, still use lard as a lubricant).
The trick is to stock up on Coke during passover. Look for the circle-U on the can. Corn syrup isn't kosher for passover, so for a few weeks each year, the company temporarily switches back to beet or cane sugar, like in the old days. Connoisseurs swear they can taste the difference.
(As an aside, I wonder if that's going to be another side benefit of the push for ethanol -- as demand for corn rises, maybe sugar beets or imported sugar will become more economical than corn syrup, and we'll go back to real sugar in our sofas again ... we can hope.)
Um, make that real sugar in our sodas again. When I spill my Coke in the sofa, it really doesn't matter what sweetener it has.