For instance, when I start a movie in Netflix and it says retrieving, am I retrieving the whole movie? If I shut the movie off after two minutes will it have charged me data usage for the whole movie or just for the time that I actually watched it?
On the original Roku, when you start playing a title, the Roku would fetch a few minutes of the film and will fetch the thumbnail file. Depending on the stream negotiated, it could hold 1 to 5 minutes of the film, probably around 2 to 3 minutes for a 4-dot stream (highest quality Standard Definition stream). The advantage of buffering ahead a little is that sub-minute Internet congestion generally will go unnoticed. (I had even rebooted my cable modem and router while watching a film and the Roku not have a hiccup, though I normally don't recommend this.) While playing, the Roku would generally play from its buffer and when there is room for another minute or so, it will again fetch more of the stream. My Roku, with a 4-dot stream, after the initial minute or so of fetching enough to fill up its buffer, will generally go about 40 seconds playing from what it has without touching the Internet, then spend the next 20 or so seconds getting more of the stream, then not touch the Internet for another 40 seconds, etc., until the end of the movie or I stop watching the stream.
I don't know how much the Roku 2 buffers ahead, but I suspect it is at least a minute and may be several minutes.
So if you watch two minutes of a movie and stopped watching, assuming 4 dot stream and 10Mbps download rate, on the original Roku you would likely have pulled down 4 to 5 minutes of the stream. I would expect similar on the Roku 2 (maybe a little more content), but, as I said, that is just a guess. If someone has a Roku 2 and doesn't mind interrupting the Internet connection while watching the movie, one could determine this (though it probably depends on several factors, including how much of the buffer had been depleted and not yet refilled when the connection was broken).
Warning: if the Internet connection is interrupted, Netflix doesn't know how much of the buffer you had watched, so your placemark on that title will be off.