Recently, Adobe introduced a new iPhone/iPad app called VideoBite. It is quite easy to use. Below is a very simple test video I made with it, and then uploaded it to YouTube from within the app.
You can also add transitions between multiple clips, some selected video filters/effects, and background music. If you add background music it replaces the audio on the video. You can mix photos and video clips to make your final video.
You can share your video in a number of ways, in addition to YouTube. It is added to your Camera Roll on an iPad. The iPhone/iPad version is available in the Apps Store.