I record my audio with GarageBand and my video with a Fuji X100 camera.
From GarageBand, I send the audio file to iMovie.
I then import the movie file using an SD card adaptor.
From iMovie, I can select the movie file and manually synchronize the two sources.
Here's an example of a video I produced using iMovie:
http://youtu.be/iisVG2s1p8I
iMovie for iPad is pretty limited and it has a number of annoying bugs -- but what it does it does reasonably well. It's also dirt cheap and super fun to use.