I actually get this film in the top of my 5 gallon too! I was just about to post about this.
My tank, however, does have a filter and the film appears within a short time after water changes. I have 4 other tanks and his is the only one where the film appears. I use spring water for all my water..What could it be? =(
I don't know if this is too late or not - it's too late for me to read all the posts. I recently had the same problem in both my tanks this weekend. Synthisis and I drained the water and scrubbed the walls. Found out it can be caused by an excess of Iron in the water and can be caused by dirty tank walls. After cleaning the tank walls really well it's been fixed and there is no film on the water now.
Edit: I should mention both tanks are 5 gallon Eclipse corner 5's with a filter and live plants.