Hi Bunny and Ripley! Welcome to PT! Like everybody else so far, I have no idea why some cats have a mysterious attraction to plastic. I was actually surfing around some pet sites yesterday and found this from a vet:

".... It is a common problem that I see all the time, and one that I myself battle with my cat Shea. You cannot imagine how many times I have had to go in surgically to remove plastic from a cat's stomach/intestines (one memorable time I removed a handful of plastic Easter grass from a cat's stomach - four months after Easter!!) She is not lacking anything in her diet, she is not unusual, she is not sick. She is a cat, and that is about the best explanation that I can give. As to how to stop it, a couple of suggestions. First, this could be a boredom problem. Give her some other toys to play with, or maybe even another cat as companionship. Secondly, and most important, cat-proof your house."

OK, so it doesn't really tell you anything you didn't already know, but it's always nice to know you aren't alone!

Laura