Comcast main service description if you search by your zipcode and look at the "Performance" plan:
"Get download speeds up to 12 Mbps and uploads up to 2 Mbps with PowerBoost® for only $19.99 a month for 6 months!"
Click to immediate link in the summary "Terms and Conditions", in the first paragraph:
"Actual speeds vary and are not guaranteed. PowerBoost provides bursts of download and upload speeds for the first 10 MB and 5 MB of a file, respectively."
You will find similar clauses in any service description for residential cable/dsl.
I am all for pushing the companies to get the best service for the cheapest price, but you do that by voting with your dollar. It is not reasonable or effective to agree to a service contract, then demonize the business for living up to their end of the bargain as they said they would.
The bottom line is you didn't pay for those speeds for any guaranteed amount of time. Techincally those speeds aren't guaranteed at all if your line quality isn't good enough to support the transmission. The only way to get guaranteed service is to pay for a dedicated line, ie: T1.
Go look at the cost of a T1 and realize it is only 1.5 Mbps, now look at the cost of that 12 Mbps residential cable. Why do you think the T1 is so much more expensive for almost 1/10th the speed? Technology may change, bandwidth may get so cheap it doesn't matter, but right now, guaranteeing 100% throughput at residential service prices simply wont work.