I created a pdftk homebrew tap:
brew tap spl/pdftk
brew install pdftk
The current formula depends on a specific compiler (gcc48) with a specific build flag (enable-all-languages). This is only be a problem if you happen to already have gcc48 without the flag or if you have a different version of gcc. You can, of course, clone the formula and change it for your situation. But, unfortunately, nobody has answered my question about alternative dependencies, so I don't know how to resolve this.
Update 1 (2015-03-19): As of this change (described in my solution to alternative dependencies), the formula now supports multiple versions of gcc and different flags.
Update 2 (2016-01-12): For OS X 10.11 El Capitan users, the source build of pdftk does not seem to work. There is a .pkg available at https://stackoverflow.com/a/33248310/545794 . As of this date, PDF Labs has not yet released a source update that fixes the build.
