iOS 5.1 gives the ability to delete from PhotoStream. Here is the initial impression I found.
I dont think it deletes photos from PhotoStream as a whole, only the PhotoStream that is for that device.
Example: I deleted a photo from my iPad's PhotoStream. It specifically said this photo will only delete from my photostream on my iPad.
I opened my iPhone and confirmed it. Did another test deleting one photo from my iPhone's PhotoStream. Same message popped up stating this would only delete the photo from the Photostream on this iPhone.
So I'm assuming you'll have to manually delete the unwanted photos each time on every device.