As I understand, some ISP DSL providers such as Covad, Northpoint, etc... go to your home and install a brand new dedicated wiring from the telephone pole to your home exclusively for your DSL line. Those companies usually use their own facilities, switches, and cabling, which is why they tend to be more expensive than the average (but then again, their main focus is on providing service to businesses and residence customers who cannot receive DSL from their local service providers [because they exceed a particular distance from their central office that DSL needs to work properly)
Then you have your Local Telephone service providers (i.e. Pacific Bell, Verizon, etc...), they provide their own service using their own telephone lines, and for the most part you have the advantage that if you happen to use their "filter" system for the rest of the phones sharing the DSL telephone number, then you can move your computer from one room to the next, plug in your modem to the line (without the filter of course) and you are connecting to the internet with DSL speed with no problem. Of course the phone company would own their own equipment.
Lastly you have your Third Party DSL providers (i.e. AOL, MSN, Prodigy, Earthlink, etc...), those providers tend to use the local service providers system, therefore being capable of giving you the exact same speed as the local service providers would give you. The only difference would most likely be the TECH support. However should you need technical assitance at your home, don't be surprise if your local telephone service technician showed up to your door step to troubleshoot your DSL problem. The key word is "capable" because these companies may ask to limit the transfer bandwith to 700kb/s or 500kb/s or whatever their breakdown is, simply to be more competitive in pricing.
P.S.
I am not aware of Third party providers contracting actual DSL providers such as Covad and Northpoint, etc... for service. So I assume they only provide DSL service to whomever would qualify under the guidelines that the local service provider has.