This one isn't as much of a hack as it is a feature of the Netflix Watch-Now (now called watch-instantly) website. Netflix will monitor for movies you have in your queue to see if any of them are currently available with the free streaming, Netflix Watch-Now selection.
For instance, I had the movie, "The Good German" in my Queue for movies I wanted to rent from Netflix. When I went to the Watch-Now page on Netflix, It showed me that "The Good German" movie was available on watch-now as you can see from the screen-shot below.
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Also be sure to check out the excellent Hacking Netflix Blog to keep up with news and tips regarding Netflix