Netflix routinely adds films to Instant Watch that it will only keep for N months, presumably to keep an interesting stream of new titles available while saving costs on licensing.
Yup. I cache the API results for 7 days with a random chance of re-poll so I hopefully will notice when new releases happen.
As for the parents, yeah, sorry :( I'm doing a match on the netflix title. I'll try to suck more data out of IMDB and make sure it is the correct movie.
Content owners restrict it, for example if a movie is about to be shown on television it will disappear from NetFlix's instant streaming service a week or two beforehand and re-appear a month or so afterwards.
If you use Twitter, you can follow @queuenoodle (http://twitter.com/#!/queuenoodle). It'll let you know when titles are expiring about a week in advance.
