roger
putting aside the point that making movies is really hard work
Its definately more involved than just pointing a camera and hitting record.
I have a 5D Mk2 and have done some experimenting with it
this was shot in 5 mins using a contax 50mm - editing probably took an hour after that
http://vimeo.com/9156040
things to consider
1) Video uses the liveview - so you have to compose and focus using the LCD. This can make for awkward handholding of the camera - shakey video is very distracting.
2) If you are looking to record sound - I would recommend an external mic - Rode do an excellent stereo VideoMic that will fit on the hotshoe - I am sure there are others. Also make sure you upgrade the camera firmware to the latest version which offers useful audio level meters.
this was done with the Rode mic - actually the footage here is from the 5D and the Olympus E-p1 - i wanted to see how the footage from the cameras could mix - excuse the guitar playing
http://vimeo.com/9618688
3) Think about how you are going to edit the footage - the file format is tricky to edit right out the camera. The output is in a H264/MPEG 4 format which is compressed in such a way that unlike a traditional film, each fame is not captured in its entirety, but rather i captures a full keyframe and then just the differences until it needs a new keyframe. What this means is that when you come to editing if you want to say cut on frame 997, the computer has to look back to the last keyframe and calculate all the changes before letting you see the frame - this can tax a computer
I work on a Mac and have access to Final Cut, and my workflow is to convert to a larger ProRes 422 format which does have a complete image for each frame and allows for faster editing and real time visual effects. You may want to look into this aspect a little more
4)you could shoot just with the 50mm , but the zoom is probably the better option - not that you should zoom while filming, but it just offers more framing opportunities. F4 should be plenty fast as the canon produces very acceptable results all the way up to 1600 ISO
I don't know what your experience level is with shooting video, but this online resource is probably worth a look at - scroll to the bottom to find the "Start Module 1" button
http://www.bbctraining.com/onlineCourse.asp?tID=5914&cat=2781
I actually did a 4 day course at the BBC, and much of what we covered is in this online resource. Obviously it geared at program making, but a lot of the basic principals are covered
I am sure there a thousand other things i should be saying - but hopefully this will get you started
cheers
kevin