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In theory, it could be used with any broadband intenet connection, provided the necessary software was available for connection. It remains to be seen whether or not Sony - or a third party - will release software for existing broadband networks such as NTL, or whether users will be restricted to Sony's own network when it becomes available.
Personally I think Sony's best option, to gain wide acceptance of the add-on, will be to allow its use on any network, with users only needing to connect to their own secure system (when available) to download game add-ons etc.
As for the speed, I believe it is 100Mbit/second, and is probably based on existing Ethernet technology.