I'd like to point www.domain1.com, www.domain2.com, and www.domainN.com to the same fuel install and treat them differently based off where they are coming from. I'm guessing I would have to modify the router or perhaps even something before that to inspect the domain and set any application variables needed in the session to uniquely identify what domain is being used to access the app and to know what to show based off that domain. For example to show different templates and load unique data. Does this seem like a decent approach? Any better suggestions.
How to implement that depends greatly on what you want to do, i.e. what the differences are.
We have an app which provides a SaaS application, every client has his own subdomain, and the Theme class is used to provide the correct look-and-feel for the clients (the hostname is mapped to the active theme) to the correct color scheme and company logo is displayed.
In the code it's used to determine the client. Every user in the system is linked to a client via the user's meta data. A user can only login using the correct subdomain, and all tables contain a client_id column on which ORM filters when it runs queries. So a user can only see their own data.