If you want one - Bike Friday and Dahon both have a number of soft bags that would work. You could buy from either company and have the bag in short order.
You might also be able to use a large backpack rain cover from REI. Depending on your bike it might do the trick and be close at hand. But, ultimately an industrial sized garbage bag would also solve your problem and perhaps stop anyone from wanting to know what was inside...
Plus I could have a lot of fun with creepy stares and strange facial expressions when they did ask!
I've got a Strida and the bags are $100 which I don't want to spend right now. Come December though when I go to my winter home in Florida I'm going to have to take it down on the plane so I was thinking a hard case-I'd heard of using a hard sided golf club case for Stridas. I think I'd like that better than the soft bags. Anyone know anything about that?