only one of you can pay the bill!
network security implications are huge. if either of you have sensitive data on your computers or, more likely, a tendency to visit "illegal adult pay sites", who would the authorities be interested in?
you will be sharing a broadband download limit from may 2005. What if by mid-month your neighbour has used your allowance by downloading video?
the list could go on, including, what if one of you move house during the contract period?
and finally, it is a clear breach of your bt agreement. whoever pays the bill and whose line is activated is risking a great deal, the other neighbour can simply walk away from any commitments as they have no contract for broadband with bt
proceed with extreme caution