First of all, I got it working thanks for this post! LOVE IT.
I tried this several times with no results. There's a couple things I had to do to get this to work.
First of all, when I open the build prop app, and search for those two lines of code, I tap on it and it gives me two boxes. one says ro.product.model and the second box said HTC EVO VIEW 4G (or something to that effect). If I changed it to say "Nexus S" like you instructed, and rebooted, it would not save it. I would open the build prop app and it still thought it was an evo. same for ro.product.manufacturer. still said HTC. Also, I have two ro.product.model. one said HTC EVO VIEW 4G and the other had the model number.
So, what I did was uninstall Netflix.
Then, in order to get the build.prop to actually save, I had to tap the pencil in the top right, which then gives you a long list of the build.prop values, and you could tap on one, backspace and edit it. I edited both my ro.product.models to reflect the nexus S model, and the manufacturer to samsung. Instead of the two boxes, it just shows ro.product.model=HTC EVO VIEW 4G and I changed it to ro.product.model=Nexus S.
I rebooted, and lo and behold, it saved this time! Then I reinstalled Netflix (I had already tried this without uninstalling netflix and reinstalling it, and it didn't work) and viola! the cast button showed up!
So to sum it up, I recommend the following steps:
Step 1: uninstall Netflix
Step 2: download the build.prop app
Step 3: open build.prop app
Step 4: tap pencil in top right
Step 5: scroll down, find ro.product.model and ro.product.manufacturer and edit them as instructed
Step 6: reboot device
Step 7: re-install netflix
Step 8: buy the OP of this thread a beer.