I often encourage my students to aim for that role first, as it can be a more direct route into television or film (if that's the goal) than simply cutting. You won't find any want ads looking for "Editor, $200 million feature film, at least six weeks experience." You will, however, have a reasonable chance of getting a foot in the door if you are strongly prepared and able to provide both editing skills and the type of support a project will need.
Just in time for a new term for film students, a new edition of Norman Hollyn's book has arrived. It's focused exactly on the skills an Assistant Editor needs.
There's also a Kindle version, ready for instant download. I'll be adding it to my iPhone in a few weeks, when I start commuting again by train. Perfect subway reading, don't you think?