Actually, you should never, ever deinterlace film-based material! That will screw it up royally, by blending fields that belong to different frames. Deinterlacing is irreversible: mess it up, and you'll never be able to get the full source picture back. Even some so-called "professional" transfers have been messed up in this way -- just check with anyone who deals with anime encodes (e.g. half the people on Doom9).

What you want to do with video that originated from film is perform IVTC -- inverse telecine. IVTC puts the fields back where they belong, and gets rid of those bastardized BC and CD frames completely. The result is nice, clean footage with all the original frames.

Now, for a source like DV from a camcorder, there is no choice but to deinterlace. Or, rather, to either deinterlace or not. IVTC is not an option, because the footage will have been shot at either 50 or 59.94 fields per second (i.e. 25 or 29.97 interlaced frames per second).