So BT added a new security feature on the latest version of the BT Home Hub firmware (6.2.6.E at time of writing) which changes the default admin password from admin to the serial number of the router. From BT Support and Advice site:
When I first noticed this new feature I thought it was quite cool and definitely a good move from BT. [...]
Here is the second version of the ZyXEL routers penetration testing paper. This second part of the paper is also fully practical just like the first one. No theory whatsoever, but rather real juicy attacks which is what we pentesters/whitehats are interested in (after all we need to be aware of what the bad guys can do)!. Unlike the first part of the paper, this one focuses more on attack techniques rather than newly-discovered vulnerabilities. [...]