I have had things on my instant cue that have become unavailable. They usually have some notice when you look at your cue like "until 10/30/11". We have kept both types. My kids due mostly instant but DH and I find nothing of interest there and usually get DVDs.
I didn't care much for the streaming selection before so I killed the account. I think it goes to the whole contract renewal thing with production companies.
I wondered about this as well, after reading about heir difficulty in securing contracts with movie houses.
Ha, as I write this, a Netflix ad peers at me from above this 'reply' field.
I just updated our Netflix account from unlimited streaming + 1 DVD out to just the unlimited streaming and I noticed that the movie selection for streaming has decreased significantly. I know that there are movies that I watched through streaming in the past and now they are available through DVD check-out only. One movie in particular stood out.