To have the boot drive automatically mounted as read/write, create a file at the root of the drive named .readwrite; you can create it with this command:
sudo touch /.readwrite
The hidden /etc/rc boot script checks for the existence of the file, and if found, the drive is mounted read/write automatically. To create this file, you need to mount read-write first using one of the one-liners below.
Note that /Users, /etc, /tmp, and /var are mounted on the Scratch volume, which is always writable.
Problem caused by /.readwrite on Apple TV 1.1
Creating the file /.readwrite on Apple TV 1.1, can cause problems when booting the 1.1 OS. When booting, if the system concludes that a disk check is needed, the system will halt if the system volume is mounted read/write - as is the case if the file /.readwrite exists. It is thus recommended that the disk is kept read-only and only mounted for reading and writing temporarily as explained below.
Alternative One Liners
sudo mount -uw /
sudo mount -ur /
I prefer to leave it mounted read only when I'm not writing