Real world telephone data bandwidth

There is a debate in Oz at present re Telstra and Braodband data For those who not in Australia Telstra is the formerly Govt owned monopoly telephone carrier

Data speeds of up to 24MBit/s are claimed for transmission over the copper wire telephone network. This is a series of copper wire pairs from the subscribers home to a local exchange.

What is the maximum "real world" data rate achievable over such a system ? Assume for the purpose of the exercise that

home -> exchange is 3km (typical in cities) pairs are unshielded there are 100 pairs in a cable Multiple lines are active at once There is no upstream limitation - the cable is the limiting factor

I would have thought software overhead, loss and crosstalk would limit the rate to 5 MBits/s or so

Thanks for any info

Richard