Actually, not true guys...
1. PC to Router/Modem:
The software that you install to setup your service limits the speed that your pc can speak to the modem through the NIC. (hit right on the head).
2. Modem to Provider:
The majority of modems use flash so that they can update the settings (some do it nightly) in case there are any updates they can just send them out without having to mess with service. This also limits your speed (hence you tend to have the same thing as business dsl).
Your provider can limit the speed pumped to your line in the first place, notice how they have 30 differant plan's... They send the signal faster generally to ensure you have what they say you'll have, but #2 tends to deminish this.
UPIN did a lot of research with @HOME for cable and VERIZON for DSL; we actually made friends with some of the employees to get some information.
BTW, not all use usernames, some go off of the NIC on your computer's serial number for tracking, I just cloned a fake one with my router :/...