Optimized version:

# turn on

sudo rm /etc/hosts

sudo ln -s /etc/hosts{.work,}

# turn off

sudo rm /etc/hosts

sudo ln -s /etc/hosts{.play,}

Optimized further (in your ~/.bashrc):

alias work="sudo rm /etc/hosts && sudo ln -s /etc/hosts{.work,}"

alias play="sudo rm /etc/hosts && sudo ln -s /etc/hosts{.play,}"

Optimization cubed (lets you define a base hosts file):

alias work="sudo rm /etc/hosts && sudo cat /etc/hosts.base >> /etc/hosts && sudo cat /etc/hosts.work >> /etc/hosts"

alias play=...

The number of upvotes, the fact that you thought this was necessary and the fact that this application exists all made me facepalm hard. I can't even see right now.