how much should we expect carbonating to change the pH of a finished beer?

after reading the Water Primer thread and trying to reverse-engineer some popular brews, I have gotten to thinking about the final pH of a beer and how it affects your perception of taste.

So if I measure a flat sample from the carboy, then cool and force carb to say, 3 volumes of CO2, how much should I expect the pH to change due to the formation of carbonic acid?

Probably not by very much (a couple of 10th's?). While it is true that 6 grams (3 vols) of CO2 per liter would bring the pH of water to 3.6 and seems a lot of CO2 it is only approximately 0.6% by weight of the total material in the beer while the true extract is probably 2 - 3 °P i.e. 3 to 5 times that. It is the material responsible for the true extract that buffers beer pH to values that fall below 3 only in the case of very acidic beers like gueze but in the more normal case of ales to values between 4.3 and 4.6. Note that these beers are usually not carbonated to the extent of 3 volumes. Also note that beer pH is measured on degassed beer because CO2 bubbles form on the bulb and disrupt the reading. To get a true pH value for carbonated beer you'd have to do it under pressure. Another way to estimate more accurately than my guess of a couple of tenths would be to measure the pH and buffering capacity of the beer (i.e. add bits of acid and measure the pH change) and then from that calculate the pH that would correspond to the acid in 3 volume of CO2.

interesting that most finished beers fall within a very tight range 4.3 to 4.6

is this true for ciders and wines as well?

Given that beer pH is established by the yeast in order to provide an environment in which they compete favorably against competitive organisms I'd guess that the same is probably true for wines, meads and ciders with one significant distinction being that grain based wort has more buffering capacity that musts. Sounds reasonable but it is a guess.