Here are some basic notes on how I installed from source on my Ubuntu x86_64 machine.
Generally followed the build from source instructions.
sudo apt-get install ghc6 # this bootstraps so we can build ghc 7
# get latest ghc
wget http://haskell.org/ghc/dist/7.0.3/ghc-7.0.3-src.tar.bz2
tar -jxvf ghc-7.0.3-src.tar.bz2
cd there
./configure
make
sudo make install
# get cabal
wget http://lambda.galois.com/hp-tmp/2011.2.0.1/haskell-platform-2011.2.0.1.tar.gz
tar xvfz haskell-platform-2011.2.0.1.tar.gz
cd there
./configure
make
sudo make install
# get pandoc
# edit ~/.cabal/config to uncomment user-install and set the value to False (so that we get global installation)
sudo cabal install pandoc
pandoc -v