Note: Updated story with Comcast blog post instead of original source.
The Comcast Blog reports explains why Comcast's upcoming Xbox streaming video won't count against the monthly 250gb Comcast data cap.
Any XfinityTV service that travels over the public Internet, including XfinityTV.com and our Xfinity TV app on mobile devices, counts toward our data usage threshold, as they always have. The Xfinity On Demand content that we will deliver to Xbox 360 will not travel over the public Internet and is delivered in much the same way as we deliver your video service to your set top box. Your Xbox 360 essentially acts as an additional cable box for your existing cable service. As a result, our data usage threshold does not apply.
For some context: our 250GB data threshold doesn't affect 99% of our customers. You can find more details in our FAQs here. From a streaming video perspective, you can stream hundreds of hours per month, depending on the bit rate.