Look up the threads on no-chill brewing...
Also, aside from the hops, aerated wort is a delicious and near perfect bacterial medium. One of the goals of your yeast, aside from the obvious, is to outcompete the bacteria and other wild yeast, depriving possible bacteria/yeast that may have landed in your wort since coming down from boiling temps of O2 and sugars (maltose, dextrose or whatever else you used to make your brew). If your sanitation was spotless and your equipment was sterile before you added your wort then you should be okay... But it's hard in a home environment to absolutely ensure that so you may have some bacteria in or around your vessel. So overnight or even 24-48 hours MAY be okay... the no chill guys do that all the time... I'm just too paranoid and try to pitch at the end of the brew session... REgardless, everyone would agree that sanitization and good sanitization practice is so key for good beer.
Good luck!