[Wil"]

When you open a bottle of coke, or you shake the coke and then open it, you notice there are bubbles appear.
Is it because the pressure of the carbon dioxide in the coke is higher than the atmospheric pressure, and according to the ideal gas equation(PV=nRT),
the pressure decreases when the CO2 is exposed to the atm. pressure, and so the volume of the CO2 increase, then you see so many bubbles??

Or because the coke needs to have a equlibrium state of pressure with the atmosphere, it lets the oxygen to enter the coke which forces the CO2 to be out of the bottle?

[Yggdrasil]

The bubbling has to do with the solubility of CO₂ in aqueous solution.  According to Henry's Law, the solubility of a gas in a solution is directly proportional to the partial pressure of the gas above the solution (wikipedia).  As the partial pressure of CO₂ increases, more CO₂ will be dissolved in solution.

When you open a bottle of soda, you release the pressurized CO₂ in the can dropping the partial pressure of CO₂ from a very high figure to about atmospheric levels.  This in turn decreases the solubility of CO₂ in water causing the CO₂ to come out of solution.  (It's exactly like a precipitation reaction except with a gas instead of a solid.  Except chemists don't say that CO₂ precipitated from solution, we say that CO₂ evolved from the solution.)

So, it is more correct to think of it as an equilibrium.   Consider the following reaction:

CO_{2 (aq)} <--> CO_{2 (g)}

By opening the soda bottle, you decrease CO_{2 (g)}.  By Le'Chatalier's principle, decreasing the amount of product will cause the reaction to move forward, producing more gaseous CO₂ in order to reestablish equilibrium.

[Rabn]

        Yggdrasil may have left you with a little bit of an incomplete picture. You see it is true that the CO₂ is dissolved in the water, what wasn't mentioned is that the dissolved CO₂ combines with water to form carbonic acid at high pressures, the kind experienced in the production of soda.  So why isn't carbonic acid listed in the ingredients? The pKa of carbonic acid = 6.7, the pKa of phosphoric acid = 2.1(phosphoric acid is actually a polyprotic, meaning that it can put more than one H⁺ into the solution but to reduce the complexity of the explanation I'm going to ignore that fact); at the pH of  Coke, 3.4, the ratio of dissociated carbonic acid(HCO₃ + H⁺) to whole carbonic acid(H₂CO₃) is .0005, or 1 dissociated molecule out of 2000 molecules at a pH of 3.4. The ratio of dissociated phosphoric acid to undissociated phosphoric acid is 20 to 1 at pH 3.4.  Now with just a little more knowledge the picture should become clear. One thing about carbonic acid is that at atmospheric pressure it decomposes into water and carbon dioxide, check out this link it explains this process pretty well http://www.newton.dep.anl.gov/askasci/chem99/chem99661.htm.  So now let's imagine what's happening here and you'll see why carbonic acid isn't listed on the ingredients.
         They use high pressures to dissolve CO₂ into the soda after it's made. While it's under high pressure the can or bottle is capped or sealed.  Now let's open a can. Once the liquid is exposed to atmospheric pressure most of the dissolved carbonic acid decomposese, forming the bubbles of CO₂, but not all of the carbonic acid decomposes right away, only undissociated carbonic acid can decompose into CO₂ and water. Eventually all of the carbonic acid will decompose, probably within a minute or so. The liquid will reach an equilibrium, as Yggdrasil mentioned in his post, and the maximum amount CO₂ that can be dissolved as gas, at the temperature of the liquid(the colder the liquid is themore CO₂ it can hold) will be obtained. So as your coke's temperature increases the less gas it can hold and it releases more and more gas from the solution keeping those bubbles forming(one of the reasons why you burp more if the soda you drink is really cold).  Now why mention the phosporic acid? well the phosphoric acid not only allows all that sugar to be dissolved; it also keeps the pH low enough that 99.9995% of the carbonic acid remains undissociated and hence it will decompose into water and CO2. 
         So why isn't it on the ingredients there is some in the soda? In order for something to be required on the ingredients lists it needs to be present in the food in the quantity of 2% by weight. By the time the soda hits your lips after opening it the amount of carbonic acid in the liquid is not even close to 2% weight. See http://a257.g.akamaitech.net/7/257/2422/12feb20041500/edocket.access.gpo.gov/cfr_2004/aprqtr/21cfr101.4.htm, this is the labeling law, it mentions it in the second paragraph.