An very interesting video was posted on YouTube by DoubleHelixPrdctns. He demonstrates that once Comcast detects Netflix activity his connection is automatically throttled to a few Mbps. Speeds go back to normal once Netflix is turned off.
In the video the user mentions higher pings times during his test with Netflix running, but with no stream activity, and this contradicts the finding that Comcast is throttling. The act of throttling or traffic shaping by the provider does not in itself increase ping times. Pings only jump when bandwidth is already saturated or when there is a line or signal issue.
The high ping times lead to believe that some other local devices or Netflix itself may have been using the connection in the background and slowing his speed tests.
Still an interesting video that shows the difficulties consumers face with Internet technology, and in this case an Internet provider with a reputation for throttling p2p.