Ok, I know this is a really old article, but I wish to make a theory. What if there are more than 5 old gods. Maybe there are only 5 that needed to be imprisoned because they were the most powerful, but how about we assume there are 8. Here is my reasoning.

Many people have brought up that these old god locations make a star. I disagree. I feel like they make another, 3-dimensional shape.

Look at this picture: http://img854.imageshack.us/img854/5157/mapdg.png

Remember, the points portrayed in the map are not going to be completely accurately placed in that the continents could be of different sizes or in different locations that are not shown in the player's map.
In it I have marked locations with old god presence. Let me go through them:
Red: confirmed locations
Orange: probable locations (bottom one is in pandaria)
Yellow: high old god influence. Lets review this one:

Northern Kalimdor: There is that minion of the old god in darkshore. There are many quests in darkshore dealing with minions of old gods. Supposed corruption in Blackfathom Deeps. The list goes on.

Northern Eastern Kingdoms: When the highborne first landed there they said they felt an evil presence sleeping beneath the land and fled to Quel'Thalas.

Twilight Highlands: Self explanatory influence. However, it is difficult to see whether or not the old god influence is there naturally, or brought in when the Twilight Hammer decided to set up operations there. As such, it is not being considered.

Stranglethorn: I have heard that there is maybe an old god there. There are old god like skeletons there plus the trolls were able to summon Hakkar there. It is mentioned that Hakkar has some relation to the old gods, and if so, there had to be a lot of influence there for his summoning to be facilitated.

Now. Looking at my map, you could call that a hexagon with a point in the middle. But that's looking at the world on a flat map. There lies the problem. We need to think of azeroth as a spherical world. As evidenced by the globes in Ulduar, Eastern Kingdoms and Kalimdor are on opposite sides of the planet. So, ignoring the malestrom for a second, on a three dimensional mindset, those six points would make a hexagon encircling the planet in a north-south direction. Add the malestrom to that and now you have a 6-sided pyramid.

My theory is that this is the prison that the ultimate old god is contained inside of. But that would be strange. Having a 6-sided pyramid, lopsidingly placed inside of a planet? Makes no sense. So, what if there was another old god location, directly opposite to the malestrom. If you add that into the equation, you would have a hexagonal bipyramid directly centered inside of the planet. That is a much more logical and it isn't wasting all that planetary space on the back of the planet.

Now, with that shape put into a sphere, it would make it so that all those corners are the closest to the surface. This is why there is so much old god corruption in these areas. They are the closest points on the surface to the prison and as such the corruption can be more easily reaching these locations.

Just my theory.