Though Comcast appears to have stopped throttling on their former on-network/off-network basis, it seems like they still haven't given up completely on packet shaping. Take the following screenshots: here I begin downloading a video (legally, mind you).
The download bursts at roughly 4Mbps for about two seconds, then hovers around the 700kbps mark for the rest of the 19mb download.
If I go to download the next video in the series, I get another spike followed by normalization.
What clearly seems to be going on here is that Comcast has found a protocol- and IP-agnostic way to throttle data: by optimizing their pipe for smaller HTML transfers, they can still claim 6mbps downlink speeds while throttling large downloads. Figures.