I would say that this is at your discression - although the T&Cs do state that you should not connect from outside your home (excluding your garden), and connect only laptops/computers that you own.
It also states no more than 3 systems at any one time.
That said, I regularly have friends round (easier to have them with us than go out with a young child) and they sometime bring their laptops and connect to our BB to check mail etc.
I have also sat outside in the car with my pda connected up - so I guess I break them regularly
In summary - you will be breaking the Ts&Cs, but not the law. If there afre problems then NTL
could remove your service, but I would expect that unless you are running solid p2p or breaking the more serious Ts&Cs (spamming hacking etc) then I would expect that they would give you a warning
If things go sour with the neighbour, then just change the key, and they cannot use it anymore (or block their MAC address to be sure).
as ever - just my 2c