As has been mentioned above, CO2 dissolves in water to form Carbonic Acid. More CO2 in the atmosphere means more has the chance to dissolve in rain, which leads to rain that is more acidic than it naturally is which can have massive environmental effects all on it's own.
Completely agree with the above post on the CO2 equilibrium in the oceans as well.
Once upon on time we all thought that CFC's were harmless, remember. They were thought to be harmless as they were inert at sea level, and odourless and had no real effects. However once we kept pumping these into the atmosphere as by-products we found out that the higher levels of UV light strips Chlorine radicals off the molecules, which then go looking for something to react with, and what do they find? Ozone, they react with that and destroy ozone in the stratosphere. This leads to less UV light being absorbed by this ozone, which is it's primary function (read: the ozone layer), thus allowing more harmful UV light to reach the Earth's surface, which is a BAD thing.
Since CFC's have been phased out there has been a recorded drop in their presence in the stratosphere, this is scientific fact, and to ignore this is sheer stupidity.
The point I'm trying to make is that we may think that excess CO2 is all well and good now, but sooner or later we may realise it is doing more harm than we first thought. I think that as well as the CO2 tax, something should be done about reducing carbon monoxide and elemental carbon particulate pollution (In most cases this is as simple as getting the air to fuel mixture right in engines, as CO and C are formed when a fuel is burnt in limited oxygen)