Okay, I`ll explain it to you guys.
*SPOILERS*
The gunshop owner has locked himself inside his gunshop, and only he has the key.
He will not let the Survivors in unless they give him a pack of Cola from the storage close by, because he is running low on supplies. In order to get into the storage the Survivors have to open a door which will cause a panic event.
It is completely optional whetever you want to do this or not, but I guess it would be pretty smart considering it will unlock every single weapon in the game very early on into the Campaign.
Also, I believe that the gunshop owner will work familiar to the Church Guy from L4D1. Because Chet said in an interview that the Survivors would never have the option to harm normal people, and if there was an NPC walking around in the gunshop it would be possible to shoot him.
So I believe that as soon as you get the cola to the gunshop owner he will turn Infected and spawn as a random Special Infected. (Except for female Boomer and Spitter, I guess...)
@Zorlac666
Oh, so according to your logic any fps-game ever with optional sidequests are considered MMORPGs? (except for the ones you like, of course...)