Some kind of "update" on this issue:
Recently some Linux distributions are merging /bin into /usr/bin and relatedly /lib into /usr/lib. Sometimes also (/usr)/sbin to /usr/bin (Arch Linux).
So /usr is expected to be available at the same time as /.
The distinction between the two hierarchies is taketaken to be unnecessary complexity now.
The idea was once having only /bin available at boot, but having an initial ramdisk makes this obsolete.
I know of Fedora Linux (2011) and Arch Linux (2012) going this way and Solaris is doing this for a long time (> 15 years).