[This page is outdated. See this post for information ]
All BBox2 modems are delivered with the same administrative password. The web interface does not allow to change it. A telnet session to the modem is needed
telnet 192.168.1.1 login: admin Password: BGCVDSL2 rg_conf_set_obscure admin/user/0/password "your_password_here" save
from this point on, you can use you new password instead of the default one.
Update 15/11/2012: With newer firmware releases, the default password has been replaced by the serial of your BBox-2. The number is a series of 15 digits or letters printed on the back of the modem. Hence, they are now different for each modem. Congratulations to Belgacom for solving this well known security hole after only four years.