Camelbak sell tablets that change the taste of the water (much in the same way as screech but tasting nicer) and I believe that they're due to be given an NSN. Alternatively try leaving some luke warm water in there for a week and then clean completely before use. Don't know why it works but it seems to have worked in mine and everyone else's that I know.
Camelbaks have come a long way since we got issued them in the early 90's... damned things would split at the seams back them. Anyway... we used the water purification tablets to remove the nasty rubber taste from ours back then as oldbooty indicated.
i use robinsons sugar free orange juice in my HAWG why just drink boring old water its gopping warm and heres a top tip for keeping water cold buy the long icepops freeze them , keep them sealed up and pop them into your bladder refreeze when soft and reuse