I've done some research, and at first it only made me more confused than before.
Many sites say that CO2 is actually much more soluble in water than O2 is; around 30 TIMES more.
So I really couldn't think why is is that surface movement can increase O2 but decreases CO2. My guess was that it has something to do with the fact that there is a much greater amount of O2 in our atmosphere than CO2; more than 500 times as much. (http://en.wikipedia.org/wiki/Earth%2...re#Composition
But then I realized that in a case where we've pumped an artificial concentration of CO2 into our tank's water, with more surface contact that extra CO2 will be released into the atmosphere, because the concentration is much higher than the balance would be otherwise.
From what I can gather, normal CO2 levels in surface water is >10 ppm. So if you're trying to get a higher ppm of CO2 than that, more contact with the atmosphere through surface disturbance will cause the CO2 to move from the area of high concentration to low concentration and it will degas out of your water. Naturally, pressure will make a difference in how much CO2 can dissolve in your water; I believe that the closer to sea level the greater the pressure and the higher the CO2 level can be.
So if we follow this idea, if the plants in your tank has been using the CO2 in your water and you now have a concentration of CO2 that is less then the equilibrium concentration, it seems like higher surface disturbance would indeed cause the atmospheric CO2 to have a chance to dissolve into your water because your water has less CO2 than it is capable of holding at that temperature and pressure.
So from what I can gather the short answer is: yes in a non-CO2 injected tank with healthy growing plants that are using CO2, increased surface agitation and water movement should 'increase' the level of CO2 in the water by keeping that level constant instead of it being depleted by the plants.
Incidentally, I would assume it is the same principle in affect when we add O2 to the water; the fish are using the O2 and therefore there is less in the water than is possible, and adding agitation lets the atmospheric O2 dissolve into the aquarium water faster. Since the percentage of O2 in the atmosphere is so much higher than that of CO2, the equilibrium saturation of O2 in water is going to be higher, even though CO2 is actually more soluble.
Hopefully someone can let us know if I'm wrong about all this, but it seems like I'm probably on the right track, since it all seems to make sense (but who knows?).
As a side note
Tap water does tend to have lots of CO2 dissolved in it (maybe because it's under pressure?), however without having a constant steady supply that CO2 is not going to stick around long in your tank. In fact constant flushes of CO2 followed by levels of low CO2 when it leaves can cause BBA to get a foothold in low tech tanks because of the fluctuating CO2 levels. For this reason it's actually better to do less frequent water changes in a tank like this and/or let the water sit out to degas before adding it if possible.