Originally posted by: MrBond
Plastic bottles can leach a chemical called acetaldehyde into the coke, but chances are you're not going to taste it. Pepsi/Coke establish a maximum AA content that people can actually taste and they don't use any bottles with a higher AA content than that. While you might get an occasional "bad" bottle, you're won't consistently taste a difference because of the AA. Nothing else will leech out into the beverage.
Edit: AA is naturally occurring in fruits and vegetables and has a fruity taste. The taste of the coke would probably overpower it.
Now the carbonation thing might be right. There's a shelf life for Coke based on how much CO2 is left after a time period (again, triangle tests determine when the consumer can actually notice the amount of CO2 lost). Glass containers are a better barrier to CO2 than plastic, so all things being equal (initial charge of CO2, same batch of Coke, etc), Coke in glass may be slightly more carbonated after a month or so.