One database - multiple sites

Is it possible to have one database support multiple sites?

Members that join one site could chat to other members who are logged into another site?

I've noticed some sites doing this.  Just not with Dolphin.

In theory this is possible since all domain related information is stored in the header.inc.php file.

In the reall world it's not going to be that easy. You have to make sure that data (images/videos/etc) are stored in one central location that it's accessible to all domains. Cronjobs can be an issue as well. Some cronjobs can and should be executed on all domains, others only on a single domain. There also is the issue with sending out messages. Some modules queue messages to be send out. How does the script know from which site the message should be send from?

I'm sure if you would start implementing something like this you will run in to other issues as well.

http://www.boonex.com/m/Cross_Database_Profile_Synchronization

is this what you mean?