SERVICE PROVIDER COMCAST has confirmed that it will continue to throttle service for its heaviest users during periods of congestion.
Mitch Bowling a spokesman for the cable giant, said it will take action against specific users by significantly reducing their transfer rate for up to 20 minutes. Concurrently, the company plans to stop its indiscriminate throttling of specific applications that allegedly slow web traffic.
The new system, reportedly dubbed “fair share”, is being implemented to appease the FCC 's demand that Comcast alter its “unreasonable network management practices” by the end of 2008.
It should be noted that Comcast has also considered charging subscribers additional fees for heavy use. µ
Talktalk do it in UK too. I've seen it on a friend's connection. He has blistering speed, then we fired up a p2p to download Linux and the speed ground to a halt. A few moments after closing the p2p application and all was well again.
If you think your iSP is not meddling in this Internet Throttling scheme, check out this website :

http://www.cfos.de/speed/documentation/speedlist_e.shtml

The software for internet shaping listed all their customer - almost all ISP doing this :p

If the user fully utilize the bandwidth that he/she pays for, and the ISP network slows down, the ISP either upgrade their bandwidth, or dont advertise and try to sell bogus 'high speed' internet packages. That's fraud at it's finest level!
They had better not mess with corporate VPN users. I assure you if any companies IT department has to eventually learn that the connection problems were Comcast throttling the corporate employees connection there are going to be lawsuits.
After 15-18MB of transfer, Comcast throttles your download speed from previous 2.2MB/s down to ~750KB/s.