[lepton]

Where does the new hydrogen come from?

Water. Few acids have any major effect without the prescence of water.

Distilled pure water has a pH of 7, which is to say that the concentration of Hydronium ions and Hydroxide ions are both at 10^-7. It is these ions that have the majority of the acidic or basic effect.

[SAJ]

Agreed, except that at any point in time there are X available ions in a given quantity of water. Introduce some CO2, make some carbonic acid, but sooner or later the process stops dead, for there are no more free ions with which to combine.

I just phrased the question very poorly, sorry.

And, there are other factors to consider, too. If a body of water is rich in dissolved minerals, say calcium, introducing CO2 is going to form relatively more calcium carbonate, and a good deal less carbonic acid. pH might actually rise in this case, depending on mineral concentration.