Can't install inkscape on Yosemite with homebrew
Update 2017:

The preferred method to install Inkscape with homebrew in 2017 is:

brew update
brew cask install xquartz
brew cask install inkscape


Previous Answer:

Inkscape is a cask that you need to tap first before installing.

I tried a few of the suggestions given on this page, and the only thing that worked for me is a minor variation to jherran's answer:

$ brew search inkscape
$ brew tap homebrew/gui
$ brew install inkscape


At that point, I got the response Installing inkscape from homebrew/homebrew-x11 and the installation completed successfully. The result from the brew search command has changed a few times so be on the guard for it to change again and just tap the correct reply from the search command if it's not homebrew/gui next time you search.