I currently build on OS X and Linux.
On OS X:
brew install https://raw.github.com/Homebrew/homebrew-versions/master/autoconf213.rb
brew install sdl cairo
On Debian-based Linuxes:
sudo apt-get install libsdl1.2-dev libcairo2-dev libpango1.0-dev
git clone git://github.com/mozilla/servo.git
cd servo
git submodule init
git submodule update
./autogen.sh
mkdir build && cd build
../configure
make check && make
./servo ../src/test/test.html