To be fair I noticed grainy video (noise as it is properly called - assuming we are speaking of the same thing here) when watching The Following on Netflix and it was streaming at 3180 Kbps, 1920 x 1080. I had made the comment earlier tonight to my girlfriend that they could have used better cameras to shoot the scene or provide better lighting.
If this is not what you are talking about, but rather 'pixelated video', then it is strange. On the 10Mbps ADSL line I have the video does every now and again (on certain shows) drop down to a lower resolution when it has been going well on a higher resolution for a while. I have noticed the same on downloads where I'd average ~850KBps and then it would suddenly drop to ~300KBps for about 2-3 seconds.
This degradation also tends to be more frequent on Netflix when there are two streams running. Most of the time once it has reached a stable bit-rate it stays there.