Apple’s iOS 5 introduced Photostream as part of their iCloud offering - when you take photos on an iOS device they are automatically synced to iCloud and from there to other devices and Mac iPhoto libraries.
Once in a while, though, you might want to take a photo that you don’t want to copy automatically to iCloud, and currently there’s no way to do that (iOS 5.1, rumored to be out in March, does apparently let you delete photos from Photostream). I’m sure there are a variety of reasons that you might want to keep photos out of Photostream.
Here’s how you can do it today:
Put your device in Airplane mode (or at least turn off Wifi - Photostream should only sync over Wifi).
Take the photo.
Share the photo out of the photo app - text it, email it, however you want to preserve it.
Delete the photo from the photo app.
Turn Airplane mode off or Wifi back on.
And you’re free and clear; the photo will not be synced to Photostream.