hmmmm...
Hate to say it, but I think the ACLU has a point.
Look at the other side of the coin, what would you think if someone kept peering through the cracks or looked over the top of the partition while you were using a public restroom?
Would you regard that as an invasion of privacy or is that acceptable for someone to watch you taking a dump when you have specifically isolated yourself from everyone else by shutting yourself in a cubicle?
How about if they took photos of you ? It's acceptable to photograph you in public so if you have no expectation of privacy why not allow it in the public restroom?
However, even if the ACLU is right on this one it doesn't get anyone off the hook, since you can argue that your car is a private place but you can't (legally) have sex in your car in a public place, so why should a toilet stall be any different?