invalid option: --with-pg-dir=/opt/PostgreSQL/9.0

I'd bet a lot of ruby devs actually found themselves stuck in this problem. You checkout a github repo, you run a bundle install and = duh = a gem cannot install because of a missing library.
You're sure you've already installed the library or dependency or whatever, but in a different path from the standard one (in this example I'm talking about PostgreSQL installed via the graphical installer instead of the ubuntu apt repo); so you issue the command

gem install pg -v0.9.0 --with-pg-dir=/opt/PostgreSQL/9.0

And you get this error message: invalid option: --with-pg-dir=/opt/PostgreSQL/9.0

What's the problem? You need to separate options with another pair of dashes:

gem install pg -v0.9.0 -- --with-pg-dir=/opt/PostgreSQL/9.0

And everything will work.