Is it possible with Android phones (like with iOS 5) to take photos directly from the lock screen without unlocking first?
You could use WidgetLocker and have a shortcut on your lockscreen to your camera. I haven't personally tried this, but it should technically work.
With just the stock Android experience, no. That is, until Ice Cream Sandwich is released.
That's not to say that OEMs haven't added their own changes that do allow for launching the camera app without unlocking, or perhaps a clever developer making a change to the lockscreen.
There are apps in android market which provide an alternate lockscreen such as WidgetLock or MagicLocker(Free) through which you could have direct access to camera from the Lock Screen. Also with some custom ROM's based on CyanogenMod, you could add shortcuts to any app on the Lock screen.