The Alameda County (California) Water District has a page on storing water for emergencies:
http://www.acwd.org/faq.php5?category_id=57#22

They say bottled water bought from a store should be safe for several years, but should be stored in the usual cool, dark place protected from direct sunlight and fumes from petroleum products and pesticides. The bottles should be checked periodically for cracks and water loss due to evaporation.

I emailed them to ask about the storage. The answer was that heat and sun will likely cause chemicals in the plastic to leach into the water, and they don't know the effects of drinking water with those leachates in it. On a practical level, that's a conservative answer; they _don't_ know, and among the issues are how much of the chemicals it would take to cause any human damage and how much water you'd have to consume to get an amount sufficient to cause harm.

As an aside, I fill empty 2-liter soda bottles with tap water, freeze them, and use them instead of ice in my cooler for some camping trips. Saves me from having a sodden mess in the cooler, and gives us something cool to drink in the heat of the day. I haven't had any break in the maybe 10 years I've been doing it, although I leave room for expansion. I note, though, that the expansion is not entirely into the empty space - ice expands outward in all directions, so do have some care about the containers if your water freezes. You may have a cracked bottle even though there is air in the top.