I found this thread because I wanted to delete a single photo too. I'm sorry to hear that its not (yet) possible.
As soon as I thought a bit about the problem I realised it was more complicated than I first thought. So I thought I'd share my thinking if it helps explain maybe why PS can't delete photos. I don't have any inside info and I could be totally wrong.
The whole point of photostream is to create a seamless transfer of images between devices as quickly as possible with no user input needed.
Once you've established that goal you prioritise the speed of upload and no user intervention.To realise the goal of almost instant transfer up to the cloud and then back to other devices as soon as a photo is taken, the iDevice which took the image trys to start synching the photo immediately to the cloud if Wifi is on and connected. It doesn't wait for permission or kick up a dialogue box and you wouldn't want it to. That seamless, no user input necessary synching is what makes the service so nice most of the time.
But given those 2 priorities how do you stop a photo from uploading to the cloud? I don't see how you can do it without violating the original priorities. It's a tradeoff between contradictory needs.
A prioity of the cloud part of iCloud is that it's a synching system and there isn't a file repository that anyone can actually visit and manipulate, its not dropbox or a "locker" where you can just take a peek into your folder in the cloud. If that's what you want, then iCloud is not designed to meet your needs. You need to go elsewhere. Those services have different priorities, they don't try to immediately synch up a series of devices.
iCloud is about seamless invisible (to the user) synching between devices. It's not a user adressable storage area in the sky. And that means there's no way to go into iCloud itself to delete the photo.
So now you have your (unwanted) photo in the cloud and iCloud's immediate task is to synch it to all permitted devices ASAP again without user intervention. So the photo is downloaded immediately to any permitted devices which are connected to a wifi network. There's no evident way that I can think of to interrupt that process given the design goals of no user input needed for seamless synching.
Another question I thought about is which device "owns" the right to delete the photo? Only the device which took the photo? That would make a complicated scenario as iCloud held images from various devices where the image might be needeed or used by others.
Should any user of any device be able to remove images in the PS? How would that work?
Can someone's 3 year old child just delete a bunch of family prized photos by accident?
Can someone at work delete key photos that someone sharing with them still needs?
Can someone delete a photo on one device that's being viewed on another device?
I know that synching is a very complicated challenge for coders so just maybe it's not that easy to code and develop a set of heuristics to handle all the potential complications. Maybe at the risk of delaying all of iOS 5, they moved solving the problem into either an update or version 6. Nobody ever produces perfect software, these kinds of decisions have to be made all the time by engineers and admin at Apple or Google or Twitter or Amazon, etc in order to actually get a product out.
Message was edited by: All Day Breakfast. Tried to be clearer at what I was saying