I have commuted for last ~5 years using a TIMBUK2 commuter pack. The bag is advertised as "water resistant" and the bag has lived up to this. The bag will keep my stuff dry when pitted against my sweat or light rain or mist. But anything more than a drizzle and I have to take extra measures.
So I carry an extra plastic grocery bag folded up in one of the pockets. If rain is even remotely expected, I'll wrap my laptop in plastic bag and nestle between my clothes.
CJ