Don't get me wrong, I like cats.
But cats are FAR more violent creatures and killers than any snake. Feral cats in particular are responsible, either directly, or as part of other similar issues, for MANY of the endangered species we are trying to save at this point in time. Pick up the April 2007 issue of Reptiles, and read the article on iguana conservation. Almost all the iguanas listed as critically endangered became critically endangered in part due to the introduction of feral cats, and sometimes dogs, to the islands they call home.
Cats are perfectly designed killers, and what's more, they are designed to take down animals larger than themselves! Did you know that a cats front teeth are designed so as to slip between the vertebrae of mammals and cleanly sever the spine? Yikes! Now that is a killer!
I would definitely say that a cat is a far bigger danger to most small-medium snakes than a snake would be to the cat.
If you don't have a seperate room, buy a critter cage. They have sliding, lockable tops, and should be more or less cat proof. If your cat is a bit hefty, I'd try to find some way of discouraging him or her from laying on top of the screen- I am not sure my screen is sturdy enough to hold the long-term weight of a cat. :shrugs: Better safe than sorry??
Preventing escapes for me is critical because my snake stays in my classroom. If he goes missing, I will never see him again. So I use the critter cage and ALWAYS, ALWAYS secure the lid FIRST before doing anything else. Even when I am just washing out his water bowl... the lid gets secured. Good luck!