Well for one thing people pointing to AUS there's a huge difference. Getting internet to AUS is expensive since its more or less a gigantic island in the middle of nowhere. It should in no way be capped in the US which is the exact opposite. Most internet traffic comes TO the US so there's no AUS style excuses about how it should be limited because getting bandwidth to US customers is hard.
The reason this is done at all is because the cable companies had two options. The first was building a decent 21st century infastructure, but that would be expensive and cut into obscene revenue. Instead they decide to throttle big internet users by charging insane overage fees and capping downloads. Did you know Comcast pulled in 8.6 billion in just ONE quarter of 2008? The average monthly cable bill is 109.66!!!!!! They want to charge 110 dollars a month AND cap download speeds? That is corporate greed at its worst.
For those who think 250 is a lot it really isn't. I rented at least 10 movies on my apple TV and PS3 this past month in HD, thats about 15GBs per movie so theres 150GBs gone. I routinely back up to an external storage system (my idisk) so thats a lot up and down as I move things to my internet storage (at least another 30 GBs over the month), I watch Hulu a lot since I don't have cable TV, they broadcast in HD as an option so thats at least 2 or 3Gbs per show so 20 shows over a month is another 60GBs and poof, I'm already over.
That is without any bittorent downloading at all, there is just a lot of content available for streaming and for HD. Another thing comcast is doing here is throttling the cable free lifestyle. I only have cable internet because I can get shows from cable channels (FX shows are available for streaming viewing free on Hulu as are shows from USA and many other cable networks) free, I can watch events like the olympics free online with much better selection (you choose what event you want to watch and when)...really there is almost nothing except a few sports you can't watch online easily and free now. For the rare events that are hard to watch free online like football games, there are corner bars where a 3 dollar drink costs loads less then a 110 dollar cable bill.
The big problem is the cable monopoly system. Allowing only one cable and one DSL provider into a market in the US is stupid beyond all belief. I would rather get FIOS in San Diego but can't because ATTs crappy 3mb/s DSL is the only game in town. Time Warner is looking to cap too because they are the only cable game in their markets. Lack of competition is death to consumers pocket books.