Act fast, or you're sharing
The first step was easy: I stepped outside and took a picture of Pittsburgh's scenic Point State Park. Then I walked back inside within range of Wi-Fi. Looking at my Photos app, I noticed there was now a new album, called Photo Stream. All the new photos you take are moved here silently and automatically (though not instantly, as I'll discuss in more detail momentarily). And yes, I said "new photos," which should come as a relief to anyone who was worried that iOS 5 would start sharing the hundreds of photos they already have on their device.
Photo Stream is what gets shared to any iOS 5 enabled device that you've registered to iCloud with your Apple ID. And here's the biggest annoyance about it if you're a control freak like me: You can't edit it. You can purge it, by going to icloud.com, entering your Apple ID, clicking on your name at the top right of the page, clicking "Advanced," and then choosing "Reset Photo Stream" (currently the only advanced option available). This will clear out the Photo Stream folder on all your devices, but not erase the pictures themselves.
I'm sure that everyone reading this can now think of multiple ways this could be annoying. Here's one that's been on my mind while I writing this: if you're taking a bunch of screenshots on your phone because you're writing a blog post about iOS 5, you're going to end up with a bunch of screenshots in your Photo Stream. Another problem I foresee that might be encountered by ordinary humans: when I'm out shopping with my wife and I catch her lingering over something without buying it, I take a surreptitious photo of it with my phone so that I can add it to my list of potential birthday or Hanukkah gifts. Since we share an Apple ID, that move will get a lot less slick when those photos start popping up in her phone's Photo Stream.
There is a solution to the latter scenario. Pictures don't get moved into your Photo Stream until your phone is connected to Wi-Fi. So, I could just email those gift pictures to myself and then delete them from my phone before returning to my warm blanket of wireless connectivity. In fact, even if you are in Wi-Fi range, you still have a minute or so delete awkward/bad/compromising photos before they start beaming out to everyone you share an Apple ID with.