This method is approved by the World Health Organization for treating household water. Here's a link:
For the lazy, here's a link to some 30+ studies that confirm light filtering through clear bottles (like PET plastic) will disinfect water a significant amount over 6 hours: http://www.sodis.ch/methode/forschung/publikationen/index_EN#health
Edit: Here's some more info.
-Even extreme levels of fecal matter (100,000 E. Coli per 100ml) are rendered harmless by UV exposure using the SODIS method (Solar Disinfection).
-A bottle 3 liters or smaller must be placed on its side for 6 hours in direct sunlight to be rendered harmless.
-Only clear bottles will correctly allow UV radiation in.
-Some bacteria grows with sunlight, but this is harmless bacteria.