The project I'm currently working on is similar in that respect; one account for multiple sites.
The solution I've gone for is delegated authentication, one service that solely deals with authentication and issues verifiable assertions about an identity. This can be coupled with authorization as well. To get an idea of what is required you can look at the OpenID project, and OAuth2 for authorization.
Setting up this service is not easy (though the OpenID site has setup guides for it), but it does provide the flexibility to move your environments out to different physical locations without having to change your code. In fact, you're free to even keep a single database and perhaps move one environment to a dedicated machine when it gains more traction than the others.