PVC fumes are seriously toxic, as in potentially fatal. Its actually becomming a big issue because so much is used in buildings and in the case of fire people can die from the PVC fumes long before the fire or smoke kills them. I've done a lot of work with PVC and will often heat it up to make it soft and bendable. Whenever I do I use a big exaust fan, make sure the area I'm in is really well ventilated, and wear a mask.

Depending on how you design your bong you may or may not have a problem. But why chance it. Find some acrylic tubing instead. Its a little harder to work with but will be a lot safer. Plus the clear acrylic will look a lot better than white PVC. Look in the phone book under plastics and start calling.