The thing is sources.list often contains things other than the debian mirror and it may be tricky to figure out what is the debian mirror and what is something else.The solution Sven has proposed can be improved, without all the mirror bloat, I think. The sane thing to do would be for debootstrap to default to what's in /etc/apt/sources.list when no mirror is given. This avoids the logistical nightmare of having to make and update $arch.ftp.debian.org foreach architecture. If you're using it in sources.list, it has your architecture.