I just recently reinstalled my MacBook Pro, this time with Snow Leopard.
So I'm again going through various installation problems I already forgot about from few years back when I installed Leopard.
Anyway, just had to hunt down a problem with mysql gem installation on Snow Leopard.
Apparently if you just do 'gem install mysql' it seems to install fine, but doesn't work.
You get "uninitialized constant MysqlCompat::MysqlRes" error when trying to run rails.
The solution is to pass it some arguments for installation:
ARCHFLAGS="-arch x86_64" gem install mysql -- --with-mysql-config=/usr/local/mysql/bin/mysql_config
Oh, and of course install 64bit mysql version before that. I usually just download the binaries from mysql.com.