In reply to Simon1000:
I had exactly this problem - I left a carton of milk in my sac, which leaked, and stayed unused for two months.
when I picked up the sac i got an overwhelming sense of "Milky Milky!"
what I did was strip out the frame, bung it in the gentlest wash the machine did - and it came out fine (had to repeat this as it still was milky).
It souldn't affect the waterproofing, but there's no such thing as a waterproof sac anyway right?
tip: make sure that you reinsert the frame the right way round!