Comcast is not a provider in our area, but they are following suit with Time Warner Cable and setting a downloading limit. Unlike TWC, they're not "test marketing" the limit. Instead they're rolling it out to all customers effective October 1st. Biggest difference in the two is the TWC "test marketed" limit is 5GB/month while Comcast is going with a 250GB/mth limit on all customers.
Now a 250GB limit seems reasonable. I'd guess (and this is just a guess) that I download between 10GB and 40GB during a given month, so this 250GB limit on the surface seems reasonable- on the surface at least.
It wasn't that long ago (well 3 PCs ago, but not that many years ago) that I owned a PC with a 'large' 20GB hard drive. At the time downloading 5GB/month would have been difficult to accomplish and that 5GB/month limit would have seemed quite reasonable just like this 250GB limit seems reasonable now. But at the time Xboxlive, Netflix, iTunes, MLB.tv, youtube, and the multitude of other ways to download and stream multimedia didn't exist.
I foresee the next phase of multimedia web content going towards offering better quality and higher definition and as they go that way, that media is going to take up larger and larger amounts of bandwidth. In other words, while that 250GB seems like a rather large number today, I doubt it will seem that way 5 years from now.
I'm guessing these limits are going to happen with most providers and we're just going to have to live with it, but I hope I'm wrong. I also don't think that (at least in the case of Time Warner) that these limits have as much to do with the providers' stated desire to fix their bandwidth problems and have a lot more to do with wanting to increase their profits by charging extra without explicitedly raising their rates.

I was extremely encouraged
I was extremely encouraged about seeing this, though I think it has to do a bit more with the FCC backlash over their throttling of BitTorrent traffic than a genuine desire to impose generous limits. 250 GB is a more-than-reasonable number. If you download HD movies from Xbox Live or wherever, that's 35-40 movies a month which seems reasonable to me. If you download more than that, chances are you are also able to afford whatever "overage" that may get charged.
Should that limit stay frozen at 250? Absolutely not. I'm hoping the number rises in future as more rich content becomes available online.
I always thought the 5 GB/month limit was absolutely ridiculous. Really anything short of 100 seems low.
I'm more of a pessimist than
I'm more of a pessimist than you.
I see this 250GB cap as a "foot in the door" that will lead to a more restrictive cap later and/or a tiered cap service with the 250GB being an increased more costly tier at some point.
Hope I'm wrong.
With TV shows being streamed
With TV shows being streamed online and quality increasing this could easily be surpassed in a few days (if all the content is HD quality). The cable companies are investing money to limit bandwidth when it should be used to improve their networks.