I use mysql and macports on OSX and both were broken when I upgraded to Snow Leopard.
Mysql was a quick fix:
ln -s /usr/local/mysql-5.1.35-osx10.5-x86 /usr/local/mysql
(you’re installed version might be different). It turns out a symlink was removed during the Snow Leopard upgrade.
As for MacPorts, I had to install Xcode from the Snow Leopard CD, install the Snow Leopard version of MacPorts and then follow this migration guide.