Here are some interesting numbers:

Assuming a 10Mbps link, you could theoretically download just over 3TB per month at full speed, 24 hours a day. That means that Comcast is limiting you to one-twelfth of what you assumed you could get.

You've gotta figure a Netflix download is probably about 2GB. A DVD would be about 4GB, the quality is somewhat lower, and the compression is probably a little better. (I would guess H.264 vs. a DVD's MPEG-2.) So give it some overhead and call it 100 movies. That's still more than three a day, so it's probably not a significant barrier, unless you have a digital hoarding complex.

Still, it sucks for them to change the rules on you.