Is it possible to use the camera without unlocking first? Is it possible with Android phones (like with iOS 5) to take photos directly from the lock screen without unlocking first?
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.