I just had trouble installing and running Mysql on 5.1.39. Here's what worked:

brew install mysql

This printed some instructions which got mysql running just fine, but the mysql gem decided to be difficult - uninitialized constant MysqlCompat::MysqlRes. Some googling found the solution:

export ARCHFLAGS="-arch i386 -arch x86_64"
gem install mysql -- --with-mysql-dir=/usr/local \
--with-mysql-config=/usr/local/bin/mysql_config

That's assuming your homebrew prefix is /usr/local. Worked for me!