Here is a great trick for washing shower curtains. Simply put the plastic liner into the washer with 2 or 3 WHITE or light-colored towels. Fill the tub to maximum capacity with hot water. If you have a bleach dispenser on your washer, add 1 cup bleach to the dispenser; otherwise wait to add bleach. Turn on the machine and run on delicate cycle for 5-6 minutes. If you have not added bleach yet, do so now. Turn off the machine and let sit for 10 minutes, then continue washing as usual. Hang the shower curtain to air dry in the bathroom or on a clothesline. The bath towels will keep the shower curtain protected from the machine's agitator.
Yes, you surely can wash those plastic liners! Here's a cheaper trick for you, toss it in the machine (delicate cycle of course) with 1/4-1/2 cup white vinegar--the more soap/mildew buildup the more vinegar (no detergent necessary) or 1 large scoop oxy-clean and allow to go through the cycle, then re-hang. ;)
It probably works really well, but what I've done in the past with shower curtains (I assume we are talking about the white liners that actually get soap stains and mildew on them vs. the colorful ones that hang outside of the tub) is just run them through hot water wash cycle in the washing machine with regular detergent. Yup, the plastic ones... and even the ones with metal rivets in them. Fresh like snow! DO NOT EVER PUT THEM IN THE DRYER!