The homeboys at Homebrew decided to make the "htop" package unofficial, stranding many users. Here's how to get some htop goodness working again.
brew install autoconf
brew link autoconf
brew install automake
brew install --HEAD https://raw.github.com/adamv/homebrew-alt/master/unmaintained/htop.rb
chmod 6555 /usr/local/Cellar/htop/HEAD/bin/htop
sudo chown root /usr/local/Cellar/htop/HEAD/bin/htop
Uninstall htop first, then execute the commands above for a fresh install.